the history of life

Lecture Presentations by Carol R. AndersonWestwood College, River Oaks Campus

© 2014 Pearson Education, Inc.

BIOLOGY Life on Earth WITH PHYSIOLOGY Tenth Edition

Audesirk Audesirk Byers

17The Historyof Life


Chapter 17 At a Glance

17.1 How Did Life Begin?

17.2 What Were the Earliest Organisms Like?

17.3 What Were the Earliest Multicellular Organisms

Like?

17.4 How Did Life Invade the Land?

17.5 What Role Has Extinction Played in the History of Life?

17.6 How Did Humans Evolve?



Pre-Darwinian thought held that all species were simultaneously created by God a few thousand years ago

Until the 19th century, most people thought that new members of species sprang up all the time through spontaneous generation both from nonliving matter and other, unrelated forms of life



Medieval beliefs reflected the concept of spontaneous generation

– Maggots were thought to arise from meat

– Microbes were thought to arise from broth

– Mice were thought to arise from mixtures of sweaty shirts and wheat

The maggots-from-meat idea was disproved by Francesco Redi in 1668

– No maggots developed when he kept flies away from uncontaminated meat



The broth-to-microorganism idea was disproved by Louis Pasteur and John Tyndall in the mid-1800s

– Microorganisms did not appear in sterile broth unless the broth was first exposed to existing microorganisms in the surrounding environment

– Pasteur and Tyndall’s work demolished the notion of spontaneous generation



The broth-to-microorganism idea was disproved by Louis Pasteur and John Tyndall in the mid-1800s (continued)

– Pasteur and Tyndall’s work did not address the question of how life on Earth originated in the first place

– Stanley Miller stated that “Pasteur never proved it didn’t happen once, he only showed that it doesn’t happen all the time”


Figure 17-1 Spontaneous generation refuted

no growth growth

The broth in a flaskis boiled to kill preexistingmicroorganisms

The long, S-shaped neckallows air, but not microorganismsto enter the flask

If the neck is later broken off,outside air can carrymicroorganisms into the broth



The first living things arose from nonliving ones

– Modern scientific ideas about the origin of life began to emerge in the 1920s

– Alexander Oparin in Russia and John B. S. Haldane in England showed that spontaneous formation of the complex organic molecules necessary for life would not be permitted in today’s oxygen-rich atmosphere



The first living things arose from nonliving ones (continued)

– Modern scientific ideas about the origin of life began to emerge in the 1920s (continued)

– Oxygen reacts readily with other molecules, disrupting chemical bonds

– An oxygen-rich environment tends to keep molecules simple




– Oparine and Haldane speculated that the atmosphere of early Earth contained little oxygen because an oxygen-rich atmosphere would not have permitted the spontaneous formation of complex organic molecules

– Oxygen’s high reactivity with chemical bonds would have prevented large molecules from forming




– Some kinds of molecules could persist in the lifeless environment of early Earth better than others and would therefore become more common over time

– The chemical version of the “survival of the fittest” is called prebiotic (meaning “before life”) evolution




– Organic molecules can form spontaneously under prebiotic conditions

– In 1953, Stanley Miller and Harold Urey set out to simulate the first stage of prebiotic evolution in the laboratory

– They noted that the atmosphere of early Earth probably contained methane (CH4), ammonia (NH3), hydrogen (H2), and water vapor (H2O), but no oxygen




– Organic molecules can form spontaneously under prebiotic conditions (continued)

– In 1953, Stanley Miller and Harold Urey set out to simulate the first stage of prebiotic evolution in the laboratory (continued)

– They simulated early Earth’s atmosphere by mixing the gases in a flask and adding an electrical discharge to simulate lightning

– Simple organic molecules appeared after a few days





– In 1953, Stanley Miller and Harold Urey set out to simulate the first stage of prebiotic evolution in the laboratory (continued)

– The experiment showed that small molecules likely present in the early atmosphere can combine to form larger organic molecules if electrical energy is present


Figure 17-2 The experimental apparatus of Stanley Miller and Harold Urey

CH4 NH3 H2 H2O

electric sparkchamber

condenser

An electric spark simulatesa lightning storm

Organic moleculesappear after a fewdays

Energy from the sparkpowers reactions amongmolecules thought to bepresent in Earth’s earlyatmosphere

When the hot gases inthe spark chamber arecooled, water vaporcondenses and anysoluble moleculespresent are dissolved

Boiling water addswater vapor to the artificial atmosphere

boiling chamber

water

cool waterflow





– Similar experiments by Miller and others have produced amino acids, short proteins, nucleotides, ATP, and other molecules characteristic of living things





– Modern geochemists believe the early atmosphere was somewhat different from that modeled in Miller and Urey’s experiments

– Additional experiments with more realistic (but still oxygen-free) simulated atmospheres have also yielded organic molecules





– The experiments proved that electricity is not the only suitable energy source

– Other sources include heat and ultraviolet (UV) light, which have been shown to drive the formation of organic molecules in experimental simulations of prebiotic conditions





– Additional organic molecules probably arrived from space when meteorites and comets crashed into the Earth’s surface

– Analysis of present-day meteorites recovered from impact craters on Earth has revealed that some meteorites contain relatively high concentrations of amino acids and other simple organic molecules





– Additional organic molecules probably arrived from space when meteorites and comets crashed into the Earth’s surface (continued)

– When small molecules known to be present in space were placed under space-like conditions of very low temperature and pressure and bombarded with UV light, larger organic molecules were produced




– Organic molecules can accumulate under prebiotic conditions (continued)

– Prebiotic synthesis was neither very efficient nor very fast

– Prebiotic molecules must have been threatened by the sun’s high-energy UV radiation, because early Earth lacked an ozone layer





– The ozone layer is a region high in today’s atmosphere that is enriched with ozone molecules

– The ozone molecules form when incoming solar energy splits some O2 molecules in the outer atmosphere into individual oxygen (O) atoms





– The ozone layer is a region high in today’s atmosphere that is enriched with ozone molecules (continued)

– The oxygen (O) atoms react with O2 to form O3 (ozone)





– Before the formation of the ozone layer, UV bombardment must have been fierce

– UV radiation can provide energy for the formation of organic molecules, but can also break them apart





– Sites beneath rock ledges or at bottoms of even fairly shallow seas, would have been protected from UV radiation

– In these locations, organic molecules may have accumulated




– Clay may have catalyzed the formation of larger organic molecules

– In the next stage of prebiotic evolution, simple molecules combined to larger molecules

– One possibility is that small molecules accumulated on surfaces of clay particles may have a small electrical charge that attracts dissolved molecules of the opposite charge




– Clay may have catalyzed the formation of larger organic molecules (continued)

– Initially, these molecules might had formed on clay at the bottom of early Earth’s oceans or lakes, but have now become the building blocks of the first living organisms



RNA may have been the first self-reproducing molecule

– DNA was probably not the earliest informational molecule

– DNA replication requires large complex protein enzymes

– The instructions for building these enzymes are coded in DNA



RNA may have been the first self-reproducing molecule (continued)

– DNA was probably not the earliest informational molecule (continued)

– This chicken-and-egg interdependency makes DNA an unlikely candidate for self-replication

– The current DNA-based system of information storage likely evolved from an earlier system




– RNA can act as a catalyst

– RNA is a prime candidate for the first self-replicating informational molecule




– Thomas Cech and Sidney Altman (1980s), working with the single-celled organism called Tetrahymena, discovered a cellular reaction that was catalyzed by a protein, by a small RNA molecule

– Cech and Altman named their catalytic RNA molecule ribozyme


Figure 17-3 A ribozyme




– RNA can act as a catalyst

– Since Cech and Altman’s initial discovery, dozens of naturally occurring ribozymes have been found that catalyze reactions, including

– Cutting other RNA molecules

– Splicing together different RNA fragments

– Attaching amino acids to growing proteins




– RNA can act as a catalyst (continued)

– The most effective replication of ribozyme so far synthesized can copy RNA sequences up to 95 nucleotides long




– Earth may once have been an RNA world

– Discovery of ribozymes led to the hypothesis that RNA preceded the origin of DNA, in an “RNA world”

– According to this view, the current era of DNA-based life was preceded by one in which RNA served as the information-carrying genetic molecule and the catalyst for its own replication




– Earth may once have been an RNA world (continued)

– This first self-reproducing ribozyme probably wasn’t very good at its job and produced copies with lots of errors

– Natural selection acted on these errors to improve the function of these early ribozymes





– This first self-reproducing ribozyme probably wasn’t very good at its job and produced copies with lots of errors (continued)

– With increased speed and accuracy of replication, these variant ribozymes reproduced, copying themselves and displacing less efficient molecules





– Molecular evolution continued

– By some unknown chain of events, RNA gradually receded into its present role as intermediary between DNA and protein enzymes



Membrane-like vesicles may have enclosed ribozymes

– Self-replicating molecules on their own do not constitute life

– In all living cells such molecules are contained within some kind of enclosing membrane

– Chemists have shown that if water containing proteins and lipids is agitated to simulate waves beating against ancient shores, the proteins and lipids combine to form hollow vesicles



Membrane-like vesicles may have enclosed ribozymes (continued)

– Vesicles resemble living cells

– Vesicles have a well-defined outer boundary that separates internal and external environments

– Depending on composition, their membranes may be remarkably similar to that of a real cell

– Under certain conditions, they may absorb material from the external solution, grow, and divide




– Certain vesicles (protocells) may have contained organic molecules, including ribozymes, and would have been the precursors of living cells

– The membranes would have served to sequester the molecules of life and to protect them from extraneous ribozymes




– Certain vesicles (protocells) may have contained organic molecules, including ribozymes, and would have been the precursors of living cells (continued)

– After sufficient time, these protocells may have developed the ability to divide and pass on copies of their enclosed ribozymes to daughter protocells




– Certain vesicles (protocells) may have contained organic molecules, including ribozymes, and would have been the precursors of living cells (continued)

– The transition from protocell to living cell was a continuous process, with no sharp boundary between one state and the next



But did all this really happen?

– Despite a great diversity of assumptions, experiments, and contradictory hypotheses in origin-of-life research, certain facts support the central tenets

– The experiments of Miller and others show that amino acids, nucleotides, and organic molecules, along with simple membrane-like structures, would have formed on early Earth



But did all this really happen? (continued)

– Despite a great diversity of assumptions, experiments, and contradictory hypotheses in origin-of-life research, certain facts support the central tenets (continued)

– Given enough time and a sufficiently large pool of reactant molecules, even extremely rare events can occur many times

– There was ample time and space for these rare events to lead to the development of life



Earth formed about 4.5 billion years ago and was hot

– Meteorites smashed into the forming planet, and the kinetic energy of these extraterrestrial rocks was converted into heat on impact

Geological evidence suggests that Earth cooled enough for water to exist in liquid form 4.3 billion years ago

The oldest fossil organisms found so far are in rocks that are approximately 3.4 billion years old



Life arose 3.9 billion years ago in what is called the Precambrian era

Geologists and paleontologists have devised a hierarchical naming system of eras, periods, and epochs to delineate the immense span of geological time


Figure 17-4 Early Earth


Table 17-1, 1 of 4


Table 17-1, 2 of 4


Table 17-1, 3 of 4


Table 17-1, 4 of 4



The first organisms were anaerobic prokaryotes

– The first cells to arise in the Earth’s oceans were prokaryotes, cells that lack a membrane-bound nucleus

– These primitive bacteria probably obtained nutrients and energy by absorbing organic molecules from their environment

– Since early Earth lacked oxygen gas, these first cells metabolized organic molecules anaerobically



Some organisms evolved the ability to capture the sun’s energy

– Eventually, some cells evolved the ability to use the energy of sunlight for synthesis of complex, high-energy molecules

– This marked the evolution of photosynthesis



Some organisms evolved the ability to capture the sun’s energy (continued)

– Photosynthesis requires a source of hydrogen, and the very earliest photosynthetic bacteria probably used hydrogen sulfide gas dissolved in water for this purpose

– The shortage of hydrogen sulfide set the stage for the evolution of photosynthetic bacteria to use water (H2O)—Earth’s most abundant source of hydrogen




– Water-based photosynthesis converts water and carbon dioxide to energetic molecules of sugar, releasing oxygen as a by-product

– This new method for capturing energy introduced significant amounts of free oxygen into the atmosphere for the first time

– Initially, oxygen combined with iron in the Earth’s crust to form iron oxide (rust)




– Initially, newly liberated oxygen was quickly consumed by reactions with other molecules in the atmosphere and in the Earth’s crust

– Iron was a common reactive atom in the Earth’s crust, combining with the new oxygen to form iron oxide (rust)

– As a result, iron is abundant in rocks formed during that time




– Subsequently, oxygen began accumulating in the atmosphere

– Chemical analysis of rocks suggests that significant levels of atmospheric oxygen first appeared about 2.3 billion years ago




– Subsequently, oxygen began accumulating in the atmosphere

– Scientists suggest that bacteria probably similar to cyanobacteria produced the oxygen

– It is very likely that we are breathing some of the recycled oxygen molecules expelled more than 2 billion years ago



Aerobic metabolism arose in response to dangers posed by oxygen

– Oxygen is potentially dangerous to living things because of its ability to react with organic molecules, breaking them down

– The accumulation of oxygen in the atmosphere of early Earth probably exterminated many anaerobic organisms

– The increase in oxygen provided the environmental pressure for the evolution of aerobic metabolism



Aerobic metabolism arose in response to dangers posed by oxygen (continued)

– The next great advance in the age of microbes was the ability to use oxygen in metabolism

– This ability provides a defense against the chemical action of oxygen



Aerobic metabolism arose in response to dangers posed by oxygen (continued)

– Being able to use oxygen also channels oxygen’s destructive power through aerobic respiration to generate useful energy for the cell

– The evolution of aerobic metabolism was significant because aerobic organisms can harvest more energy per food molecule than anaerobic organisms



Some organisms acquired membrane-enclosed organelles

– As prokaryotes began to proliferate, some developed the ability to engulf smaller ones

– These early predators, lacking both photosynthesis and aerobic metabolism, processed their prey very inefficiently



Some organisms acquired membrane-enclosed organelles (continued)

– The ability to compartmentalize functions inside the cell through internal membrane formation greatly improved the efficiency of the early cells

– The first eukaryotes (organisms composed of one or more eukaryotic cells) appeared about 1.7 billion years ago




– Mitochondria and chloroplasts may have arisen from engulfed bacteria

– The endosymbiont hypothesis proposes that early eukaryotic cells acquired the precursors of mitochondria and chloroplasts by engulfing certain types of bacteria




– Mitochondria and chloroplasts may have arisen from engulfed bacteria (continued)

– These cells and the bacteria trapped inside them gradually entered into a symbiotic relationship, a close association between different types of organisms




– The internal membranes of eukaryotes may have arisen through infolding of the plasma membrane

– The internal membranes of eukaryotic cells may have originally arisen through inward folding of the cell membrane of a single-celled predator

– An infolding of the membrane near the point of DNA attachment may have pinched off and become the precursor of the cell nucleus





– The aerobe uses oxygen to metabolize the absorbed food and gains great energy

– The aerobe leaks some energy, probably as ATP or similar molecules, back into its host’s cytoplasm





– The anaerobic predatory cell with its symbiotic bacteria could now metabolize food aerobically, gaining a great advantage over other anaerobic cells





– Because of its selective advantage, the predatory cell leaves a greater number of offspring





– The endosymbiotic bacterium lost its ability to live independently of its host, giving rise to the mitochondrion

– The descendants of the engulfed bacterium evolve into mitochondria





– A mitochondria-containing predatory prokaryotic cell engulfs a photosynthetic bacterium

– The predatory cell and the bacterium gradually enter into a symbiotic relationship

– The descendants of the engulfed bacterium evolve into chloroplasts




– Many biologists believe that cilia, flagella, centrioles, and microtubules may also have evolved from a symbiosis between a spirilla-like bacterium and early eukaryotic cell


Figure 17-5 The probable origin of mitochondria and chloroplasts in eukaryotic cells

photosyntheticbacterium

Descendants of thephotosynthetic bacteriumevolve into chloroplasts

The mitochondria-containing cell engulfs aphotosynthetic bacterium

Descendants of theengulfed bacterium evolve into mitochondria

aerobicbacterium

An anaerobic,predatory prokaryotic cell engulfs an aerobicbacterium




– Evidence that supports the endosymbiont hypothesis includes

– Many distinctive biochemical features shared by eukaryotic organelles and living bacteria




– Living intermediates (organisms alive today that are similar to hypothetical ancestors)

– The amoeba Pelomyxa palustris lacks a mitochondria but hosts a permanent population of aerobic bacteria that carry out much of the same role



The evidence for the endosymbiont hypothesis is strong

– Modern cells, called living intermediates, host bacterial endosymbionts

– The amoeba Pelomyxa palustris harbors aerobic bacteria

– A variety of corals, some clams, a few snails, and at least one species of Paramecium harbor photosynthetic bacteria



The evidence for the endosymbiont hypothesis is strong (continued)

– The fact that modern cells host bacterial endosymbionts suggests that similar symbiotic associations could have occurred almost 2 billion years ago and led to the first eukaryotic cells


Figure 17-6 Symbiosis within a modern cell


17.3 What Were the Earliest Multicellular Organisms Like?

Once predation evolved, increased cell size became an advantage

– Larger cells could more easily engulf smaller cells and would be difficult for other predatory cells to ingest

– The larger a cell becomes, the less surface membrane is available per unit volume of cytoplasm



Once predation evolved, increased cell size became an advantage (continued)

– Therefore, it becomes more difficult for sufficient amounts of oxygen and nutrients to diffuse in and waste products to diffuse out as cells get larger



Organisms larger than a millimeter in diameter can survive only in one of two ways

– They can have a low metabolic rate and thus, not need much oxygen to import and not produce much carbon dioxide to export

– They can be multicellular and thus, maintain small cell size individually while increasing in size as a group



Some algae became multicellular

– The oldest fossils of multicellular organisms appeared about 1.2 billion years ago

– These fossils consisted of impressions of multicellular algae that arose from single-celled eukaryotic organisms containing chloroplasts



Some algae became multicellular (continued)

– Multicellularity would have provided at least two advantages for these seaweeds

– Large, many-celled algae would have been difficult for single-celled predators to engulf



Some algae became multicellular (continued)

– Multicellularity would have provided at least two advantages for these seaweeds (continued)

– Specialization of cells would have provided the potential for staying in one place in the brightly lit waters of the shoreline

– The specializations of cells would appear as rootlike structures burrowed in sand

– They would also appear as structures clutched onto rocks



Animal diversity arose in the Precambrian Era

– The oldest known unequivocal traces of animals include fossil embryos found in Precambrian deposits more than 630 million years old

– Older fossils that may demonstrate earlier existence of animals include 850-million-year-old mesh-like structures that look a lot like the collagen protein



Animal diversity arose in the Precambrian Era (continued)

– The earliest fossils of the animals themselves were of invertebrates (animals lacking backbones) collected from rocks 610 million to 544 million years old

– The oldest rock layers included fossils of ancestral sponges and jellyfish

– Subsequent rock layers revealed fossils of ancestral worms, mollusks, and arthropods




– The full range of modern invertebrate animals, however, does not appear in the fossil record until the Cambrian period of the Paleozoic era (542 million years ago)

– The Cambrian period was marked by an “explosion” in animal diversity




– Almost all of the major groups of animals on Earth today were already present in the early Cambrian era

– The apparently sudden appearance of so many different kinds of animals suggests these groups actually arose earlier, but that their early evolutionary history is not preserved in the fossil record




– Predation favored the evolution of improved mobility and senses

– Early diversification of animals was probably driven by the emergence of predatory lifestyles

– The coevolution of predator and prey favored animals that were more mobile




– Predation favored the evolution of improved mobility and senses (continued)

– The evolution of efficient movement was often associated with the evolution of greater sensory capabilities and more complex nervous systems





– By the Silurian period (440 million to 416 million years ago), life in Earth’s seas included an array of anatomically complex animals





– Examples of animals from this period include trilobites, ammonites, and the chambered nautilus (which exists today nearly unchanged from its original form)


Figure 17-7 Diversity of ocean life during the Silurian period

Silurian scene Trilobite

Ammonite Nautilus




– Skeletons improved mobility and protection

– Many of the Paleozoic animal species developed increased mobility, in part owing to the origin of hard external body coverings known as exoskeletons




– Skeletons improved mobility and protection (continued)

– Exoskeletons increased mobility by providing hard surfaces to which muscles attached

– Exoskeletons also gave support to the organism’s body and provided protection





– About 530 million years ago, one group of animals—the fishes—developed a new form of body support and muscle attachment: an internal skeleton

– They were the first vertebrates (animals with backbones)





– By 400 million years ago, fishes were a diverse and prominent group





– Over time, fish became the dominant predators in the oceans

– Fish were faster than invertebrates, with more acute senses and larger brains

– Fish became the dominant predators of the seas



After more than 3 billion years of a strictly watery existence, life came ashore

Land-invading organisms faced various obstacles

– They had to support their own weight instead of relying on the buoyancy of water

– They had to locate water instead of simply being surrounded by it

– They needed to protect their gametes from desiccation



The vast empty spaces of the Paleozoic landmass represented a tremendous evolutionary opportunity

The move on to land was particularly advantageous for plants

– Photosynthesis was far more effective freed from the light-absorbing qualities of water

– Soils had untapped sources of nutrients, whereas seawater was low in some nutrients, particularly nitrogen and phosphorus

– The land was free of predators on plants, whereas the sea was teeming with them



Some plants became adapted to life on dry land

– In moist soils at the water’s edge, a few small green algae began to grow, taking advantage of the sunlight and nutrients

– About 475 million years ago, some of these algae gave rise to the first multicellular land plants



Some plants became adapted to life on dry land (continued)

– Initially simple, low-growing forms, land plants evolved solutions to two of the main difficulties of plant life on land

1. Obtaining and conserving water

2. Staying upright despite gravity and winds




– Primitive land plants retained swimming sperm and thus required water to reproduce

– Similar to today’s mosses and ferns

– Consequently, they had to live in moist areas such as swamps and marshes or in areas with abundant rainfall




– Primitive land plants retained swimming sperm and thus required water to reproduce (continued)

– Primitive land plants developed waterproof coatings on above-ground parts, thus reducing evaporative water loss

– They also developed vascular tissues, which conducted water from the roots to the leaves and provided support




– Primitive land plants prospered during the warm, moist Carboniferous period (359 to 299 million years ago)

– This period was characterized by vast forests of giant tree ferns, club mosses, and horsetails


Figure 17-8 The swamp forest of the Carboniferous period




– Seed plants encased sperm in pollen grains

– Seed plants inhabiting drier regions evolved a means of reproduction no longer needing water

– The eggs were retained on the parent plant

– Sperm were encased in drought-resistant pollen grains carried from plant to plant by the wind




– Seed plants encased sperm in pollen grains (continued)

– When the pollen grains landed near an egg, they released sperm cells directly into living tissue, eliminating the need for a surface film of water

– The fertilized eggs developed inside seeds, providing the developing embryo with protection and nutrients





– The earliest seed-bearing plants appeared in the late Devonian period (375 million years ago)

– They produced their seeds along branches, without any specialized structures to hold them





– By the middle of the Carboniferous period, conifers arose

– Conifers are seed-bearing plants with developing seeds inside cones





– Conifers, with wind-dispersed pollen, flourished during the much drier Permian period (299–251 million years ago), while the tree ferns and giant club mosses, with their swimming sperm, largely went extinct




– Flowering plants enticed animals to carry pollen

– Approximately 140 million years ago, during the Cretaceous period, flowering plants appeared




– Flowering plants enticed animals to carry pollen (continued)

– Flowering plants are pollinated by insects and other animals

– Flower pollination by animals can be more efficient than wind pollination

– Wind-pollinated plants must produce an enormous amount of pollen to reach its target



Some animals became adapted to life on dry land

– After land plants evolved, providing potential food sources for other organisms, animals emerged from the sea

– The earliest evidence of land animals comes from fossils that are about 430 millions years old



Some animals became adapted to life on dry land (continued)

– The first animals to inhabit land were the arthropods, invertebrates with jointed appendages and an external skeleton

– This group today includes insects, spiders, scorpions, centipedes, and crabs




– The first animals to inhabit land were the arthropods, invertebrates with jointed appendages and an external skeleton (continued)

– The exoskeleton gave arthropods an advantage because it is waterproof and provides support against the force of gravity

– Arthropods dominated the Earth for tens of millions of years




– Amphibians evolved from lobefin fishes

– Lobefins had two features that enabled their descendants to colonize land

1. Stout, fleshy fins used to crawl about on the bottoms of shallow, quiet waters

2. An outpouching of the digestive tract that could be filled with air, like a primitive lung




– Amphibians evolved from lobefin fishes (continued)

– Lobefin, a group of Silurian fishes, appeared about 400 million years ago


Figure 17-9 A fish that walks on land





– The benefits of feeding on land and moving from pool to pool favored the evolution of a group of animals that could stay out of water for longer periods and move about

– With improvements in lungs and legs, the lobefins gave rise to amphibians about 370 million years ago





– The early amphibians were not fully adapted to life on land

– Their lungs were simple sacs with very little surface area





– The early amphibians were not fully adapted to life on land

– They had to obtain some of their oxygen through their skin

– Their sperm and eggs had to be deposited in a watery environment





– Although amphibians could move about on land, they were limited in how far they could travel

– Amphibians, along with the tree ferns and club mosses, declined when the climate turned dry at the beginning of the Permian period about 299 million years ago




– Reptiles evolved from amphibians

– A group of amphibians adapting to a drier climate evolved into reptiles




– Reptiles evolved from amphibians (continued)

– Reptiles exhibit three major adaptations to life on land

1. Shelled, waterproof eggs that enclose a supply of water for the developing embryo

2. Scaly, waterproof skin that helps prevent the loss of body water to the dry air

3. Improved lungs





– As the climate dried during the Permian period, reptiles became the dominant land vertebrates

– Amphibians were relegated to the swampy backwaters where most currently remain





– Very large reptiles (dinosaurs, specifically) evolved during a period when the climate became wetter again

– The variety of dinosaur forms was enormous—fleet-footed and ponderous, predators and plant eaters





– Dinosaurs flourished for more than 100 million years

– The last dinosaurs became extinct around 65 million years ago

– The reason for extinction of dinosaurs remains unknown


Figure 17-10 A reconstruction of a Cretaceous forest




– Reptiles gave rise to mammals

– Unlike the egg-laying reptiles, mammals evolved live birth and the ability to feed their young with secretions of the mammary (milk-producing) glands

– Ancestral mammals developed hair, which provided insulation




– Reptiles gave rise to mammals (continued)

– The earliest fossil mammal unearthed thus far is almost 200 million years old

– Early mammals coexisted with the dinosaurs and had distinctive skeletal features

– They were mostly small creatures




– Reptiles gave rise to mammals (continued)

– The largest known mammal from the dinosaur era was about the size of a modern raccoon, but most were far smaller than that

– When the dinosaurs went extinct, however, mammals colonized the habitats left empty by the extinctions

– Mammals prospered, diversifying into the various forms seen today



The lesson in the great tale of life’s history is that nothing lasts forever

The story of life can be read as a long series of evolutionary dynasties

– With the rising of each new dominant group comes ruling the land or seas followed by decline and extinction

– Dinosaurs are the most famous of the fallen dynasties



Despite extinction, the overall trend has been for species to arise at a faster rate than they disappear

The number of species on Earth has tended to increase over time



Evolutionary history has been marked by periodic mass extinction

– Mass extinction occurs when large numbers of species disappear within a relatively short time

– The most catastrophic mass extinction was the Permian extinction (251 million years ago)

– More than 90% of the world’s species were wiped out, destroying entire ecosystems

– Life became perilously close to disappearing altogether



Evolutionary history has been marked by periodic mass extinction (continued)

– Climate change contributed to mass extinctions

– When the climate changes, organisms that are adapted for survival in one climate may be unable to survive in a drastically different climate

– Warmer climates gave way to drier, colder climates with more variable temperatures, potentially causing some species to become extinct




– Climate change contributed to mass extinctions (continued)

– One cause of climate change is the shifting positions of continents called continental drift

– Continental drift is the result of plate tectonics, the theory that the Earth’s crust is divided into irregular plates that rest on a fluid layer and converge, diverge, and slip past one another




– Climate change contributed to mass extinctions (continued)

– As the plates wander, their positions change in latitude

– North America was located at or near the equator more than 340 million years ago

– As time passed, plate tectonics shifted the continent up into temperate and arctic regions


Figure 17-11 Continental drift from plate tectonics

340 million years ago

Eurasia

135 million years ago

Present225 million years ago

Africa AustraliaIndia

Antarctica

NorthAmerica

SouthAmerica

Eurasia

Africa

AustraliaIndia

Antarctica

NorthAmerica

SouthAmerica

Eurasia

Africa

AustraliaIndia

Antarctica

NorthAmerica

SouthAmerica

Europe

Africa

Australia

India

Antarctica

NorthAmerica

SouthAmerica

Asia

PANGAEA

LAURASIA

WESTGONDWANA

EASTGONDWANA




– Catastrophic events may have caused the biggest mass extinctions

– Geologists have found evidence of massive volcanic eruptions

– However, even if enormous, such eruptions would directly affect only a relatively small portion of Earth’s surface




– Catastrophic events may have caused the biggest mass extinctions (continued)

– In the early 1980s, Luis and Walter Alvarez proposed that the extinction event of 65 million years ago, which wiped out the dinosaurs and other species, was caused by the impact of a huge meteorite




– Catastrophic events may have caused the biggest mass extinctions (continued)

– Researchers have identified the Chicxulub crater, a 100-mile-wide crater buried beneath the Yucatan Peninsula of Mexico, as the impact site of a giant meteorite—6 miles in diameter—at the time of the dinosaurs’ disappearance



Scientists are intensely interested in the origin and evolution of humans

Fossil evidence of human evolution is comparatively scarce and open to a variety of interpretations



Humans inherited some early primate adaptations for life in trees

– Humans are members of a mammal group called primates, which includes lemurs, monkeys, and apes

– The oldest primate fossils are 55 million years old

– Because so few primate fossils have been found, far older primates may have existed, but no record was left



Humans inherited some early primate adaptations for life in trees (continued)

– The earliest primates are believed to have fed on fruits and leaves, and to have been adapted for life in the trees

– Many of the adaptations are shared by primates across geological time


Figure 17-12 Representative primates

Tarsier Macaque

Lemur




– Binocular vision provided early primates with accurate depth perception

– Two large, forward-facing eyes with overlapping fields of view permitted early primates to gauge distances as they moved through the trees

– Color vision was another key adaptation because it helped primates to distinguish ripe fruit from green leaves




– Early primates had grasping hands

– Early primates had long, grasping fingers that could wrap around and hold onto tree limbs

– The grasping primate hand evolved in humans to provide both precision grip (used for delicate maneuvers such as picking up small objects and sewing) and power grip (used for powerful actions such as thrusting with a spear or swinging a hammer)




– A large brain facilitated hand-eye coordination and complex social interactions

– A large brain would have been an adaptive advantage during complex locomotion through trees requiring precise hand-eye coordination

– If the sociality seen in primates promoted increased survival and reproduction, the benefits to individuals of successful social interaction might have favored the evolution of a larger brain



The oldest hominin fossils are from Africa

– Hominins include humans and extinct human-like primates

– Comparisons of human DNA with that of apes suggest that the two diverged between 5 and 8 million years ago

– Sahelanthropus tchadensis lived more than 6 million years ago

– Sahelanthropus exhibits human-like and ape-like characteristics


Figure 17-13 The earliest hominin



The oldest hominin fossils are from Africa (continued)

– Ardipithecus ramidus and Orrorin tugenensis lived between 4 million and 6 million years ago

– Knowledge of these earliest hominins is limited because few specimens, and only partial skeletons, have been found

– The first well-known hominin line, the genus Australopithecus, arose about 4 million years ago




– Early hominins could stand and walk upright

– The earliest australopithecines (collectively, the various species of Australopithecus) possessed knee joints that permitted bipedal (upright, two-legged) locomotion

– Four-million-year-old fossilized footprints, discovered by Mary Leakey, show that early australopithecines sometimes walked upright




– Early hominins could stand and walk upright (continued)

– An upright stance was significant in the evolution of hominins because it freed their hands from use in walking

– Later hominins were able to carry weapons, manipulate tools, and perform other cultural revolutions produced by modern Homo sapiens




– Several species of Australopithecus have emerged in Africa

– Australopithecus anamensis was unearthed near an ancient lake bed in Kenya from sediments that were dated as between 3.9 million and 4.1 million years old




– Several species of Australopithecus have emerged in Africa (continued)

– The species was named Australopithecus anamensis by its discoverers

– Anam means “lake” in the local Ethiopian language





– Australopithecus afarensis was discovered in the Afar region of Ethiopia

– Fossil remains of this species as old as 3.9 million years have been unearthed





– The A. afarensis gave rise to at least two distinct forms

1. A. africanus was small, about the same size as afarensis, and was, like afarensis, an

omnivore

2. A. robustus and A. boisei were larger than their forebears and were herbivorous





– All australopithecines were extinct by 1.2 million years ago

– Before disappearing, one of the species gave rise to a new branch of the hominin family tree, the genus Homo



The genus Homo diverged from the australopithecines 2.5 million years ago

– Homo habilis appeared 2.5 million years ago

– The bodies and brains of H. habilis were larger than those of the australopithecines

– However, H. habilis retained ape-like long arms and short legs

– Homo ergaster appeared 2 million years ago

– This species had limb proportions more like those of modern humans



The genus Homo diverged from the australopithecines 2.5 million years ago (continued)

– Homo ergaster was the common ancestor of two distinct branches of hominins

– H. erectus was the first hominin species to leave Africa, approximately 1.8 million years ago

– H. heidelbergensis, in turn, split into two branches

– Some migrated to Europe and gave rise to H. neanderthalensis

– Those remaining in Africa gave rise to H. sapiens (modern humans)


Figure 17-14 A possible evolutionary tree for humans

millions of years ago

A. robustus

A. boisei

A. africanus

H. erectus

Homo ergaster

H. floresiensis

H. neanderthalensis

H. heidelbergensis

Australopithecus afarensis

H. sapiens

H. habilis

A. anamensis

Ardipithecus ramidus

Sahelanthropus tchadensis

Orrorin tugenensis

6 5 4 123 0



The evolution of Homo was accompanied by advances in tool technology

– Homo habilis produced fairly crude chopping tools that were unchipped on one end to hold in the hand

– Homo ergaster produced finer tools that were typically sharp all the way around the stone

– Some of these may have been tied to spears

– Homo neanderthalensis produced exceptionally fine tools with extremely sharp edges made by flaking off tiny bits of stone


Figure 17-15 Representative hominin tools

Homo habilis

Homo ergaster

Homo neanderthalensis



The evolution of Homo was accompanied by advances in tool technology (continued)

– Neanderthals had large brains and excellent tools

– Neanderthals appeared in the European fossil record about 150,000 years ago

– 70,000 years ago, they had spread throughout Europe and western Asia

– 30,000 years ago, the species had become extinct




– Neanderthals had large brains and excellent tools (continued)

– They were similar to modern humans

– Neanderthals walked fully erect

– They were dexterous enough to manufacture finely crafted stone tools

– They had brains slightly larger than humans




– Neanderthals had large brains and excellent tools (continued)

– Despite the physical and technological similarities between Neanderthals and H. sapiens, there is no solid archaeological evidence that Neanderthals ever developed advanced culture that included music, arts, and rituals




– Neanderthals and Homo sapiens may have interbred

– Dramatic evidence supporting the hypothesis that H. sapiens and H. neanderthalensis were separate species comes from researchers who have isolated DNA from Neanderthal skeletons

– Extraction and isolation of an entire Neanderthal genome from 38,000-year-old bones found in a cave in Croatia




– Neanderthals and Homo sapiens may have interbred (continued)

– Sequence comparison revealed that up to 4% of a modern non-African human’s DNA is similar to distinctively Neanderthal sequences

– This finding suggest that our ancestors interbred with Neanderthals approximately 60,000 years ago




– Neanderthals and Homo sapiens may have interbred (continued)

– Because modern Africans do not carry the Neanderthal sequences and all other people do, interbreeding must have occurred after H. sapiens had left Africa but before modern humans spread around the world



Modern humans emerged less than 200,000 years ago

– Fossil record shows that anatomically modern humans appeared in Africa about 160,000 years ago and possibly as long as 195,000 years ago

– European and Middle Eastern H. sapiens appeared about 90,000 years ago and were known as Cro-Magnons



Modern humans emerged less than 200,000 years ago (continued)

– Cro-Magnons were more sophisticated than the Neanderthals

– Artifacts from 30,000-year-old Cro-Magnon archaeological sites include elegant bone flutes, graceful carved ivory sculptures, and evidence of elaborate burial ceremonies




– Cro-Magnons were more sophisticated than the Neanderthals (continued)

– The most remarkable accomplishment of Cro-Magnons is the magnificent art left in caves in places such as Altamira in Spain and Lascaux and Chauvet in France


Figure 17-16 Paleolithic burial


Figure 17-17 The sophistication of Cro-Magnon people




– Cro-Magnons and Neanderthals lived side by side

– Cro-Magnons coexisted with Neanderthals in Europe and the Middle East for as many as 50,000 years before the extinction of the Neanderthals

– The genetic analyses show that Cro-Magnons interbred with Neanderthals

– Neither hypothesis does a good job of explaining how the two kinds of hominins managed to coexist




– Several waves of hominins emigrated from Africa

– The human family tree is rooted in Africa, but hominins found their way out of Africa on numerous occasions

– The H. erectus reached tropical Asia almost 2 million years ago

– H. heidelbergensis made it to Europe at least 780,000 years ago




– Several waves of hominins emigrated from Africa (continued)

– Members of the genus Homo made repeated long-distance migrations out of Africa

– According to the “African replacement” hypothesis, H. sapiens emerged in Africa about 150,000 years ago, spreading into Near East, Europe, and Asia and replacing all other hominins


Figure 17-18a African replacement hypothesis

African replacement hypothesis

Homo erectus

Homo sapiens





– Some paleoanthropologists believe H. sapiens populations evolved simultaneously in many regions from the already widespread population of H. erectus





– According to the “multiregional hypothesis”, continued migrations and interbreeding among H. erectus populations in different regions of the world maintained them as a single species as they gradually evolved into H. sapiens


Figure 17-18b Multiregional hypothesis

Multiregional hypothesis



The evolutionary origin of large brains may be related to meat consumption and cooking

– Highly developed brains may have evolved in response to increasingly complex social interactions, such as the cooperative hunting of large game

– If the distribution of this group-hunted meat was best accomplished by individuals with large brains, then natural selection may have favored such individuals



Sophisticated culture arose relatively recently

– The distinctively human characteristics made possible by a large brain include the following

– Language

– Abstract thought

– Advanced culture

– Early humans capable of language and symbolic thought would not necessarily have created artifacts that indicated these capabilities



Sophisticated culture arose relatively recently (continued)

– Biological evolution continues in humans

– The evolution of human bodies by natural selection was thought to be slowed or halted after we began to live in advanced societies

– Our ever-growing ability to rapidly sequence DNA has made it possible for researchers to analyze sequences of a large and growing number of human genomes




– Biological evolution continues in humans (continued)

– People have rapidly evolved since the advent of music, art, language, and other hallmarks of advanced culture





– Many of our genes show the telltale signs of evolution by natural selection in recent millennia

– The exact functions of these genes remain unknown, but researchers have determined the functions of some revolutionary changes

– Alleles required to digest milk developed within the past 7,000 years





– Many of our genes show the telltale signs of evolution by natural selection in recent millennia (continued)

– Researchers have identified 20 genes that have evolved rapidly and recently in Tibetans




– Culture also evolves

– Cultural evolution is the evolution of information and behaviors that are transmitted from generation to generation by learning

– The evolutionary success of humans is the result of cultural evolution and a series of technological revolutions




– Culture also evolves (continued)

– During the agricultural revolution about 10,000 years ago, people discovered how to grow crops and domesticate animals

– Because the amount of food increased dramatically, the human population rose as well, from around 5 million at the beginning of the revolution to about 750 million in 1750





– The industrial revolution gave rise to the modern economy and its attendant improvements in public health

– Longer lives and lower infant mortality led to truly explosive population growth





– Human cultural evolution and the accompanying increases in human population have made humans an important agent of natural selection with respect to other life-forms





– “We have become, by the power of a glorious evolutionary accident called intelligence, the stewards of life’s continuity on Earth. We did not ask for this role, but we cannot renounce it. We may not be suited for it, but here we are.”

Stephen Jay Gould

the history of life

Documents