12.3 Origin of Life

KEY CONCEPT

The origin of life on Earth remains a puzzle.

12.3 Origin of Life

Earth was very different billions of years ago.

• There have been many hypotheses of Earth’s origins.

• The most widely accepted hypothesis of Earth’s origins is

the nebula hypothesis.

12.3 Origin of Life

Several sets of hypotheses propose how life began on

Earth.

• There are two organic molecule hypotheses.

– Miller-Urey experiment: created organic molecules using

electrical current (lightning)

electrodes

heat source amino acids

water

“atmosphere”

“ocean”

12.3 Origin of Life

Several sets of hypotheses propose how life began on

Earth.

• Meteorite hypothesis:

– Organic molecules arrived from outer space

12.3 Origin of Life

• There are different hypotheses of early cell structure.

– iron-sulfide bubbles hypothesis:

– hydrothermal vents make sulfur + ocean water =

conditions for life

12.3 Origin of Life

• There are different hypotheses of early cell structure.

– lipid membrane

hypothesis:

– lipid molecules

spontaneously form

membrane-enclosed

spheres

12.3 Origin of Life

• A hypothesis proposes that RNA was the first genetic

material.

12.3 Origin of Life

12.4 KEY CONCEPT

Single-celled organisms existed 3.8 billion years ago.

12.3 Origin of Life

Microbes have changed the physical and chemical

composition of Earth.

• The oldest known fossils are a group of marine

cyanobacteria.

– prokaryotic cells

– added oxygen to

atmosphere

– deposited minerals

12.3 Origin of Life

• Fossil stromatolites provide evidence of early colonies of

life.

12.3 Origin of Life

Eukaryotic cells may have evolved through

endosymbiosis.

• Endosymbiosis is a relationship in which one organism lives

within the body of another.

• Mitochondria and chloroplasts may have developed through

endosymbiosis.

12.3 Origin of Life

The evolution of sexual reproduction led to increased

diversity.

• Genetic variation is an advantage of sexual reproduction.

• Sexual reproduction may have led to the evolution of

multicellular life.

12.3 Origin of Life

12.6 KEY CONCEPT

Humans appeared late in Earth’s history.

12.3 Origin of Life

Humans share a common ancestor with other primates.

• Primates are mammals with flexible hands and feet,

forward-looking eyes and enlarged brains.

12.3 Origin of Life

• Primates evolved into prosimians and anthropoids.

– Prosimians are the oldest living primates.

– They are mostly small and nocturnal.

12.3 Origin of Life

– They are subdivided into the New World monkeys, Old

World monkeys, and hominoids.

– Anthropoids are humanlike primates.

– Homonoids are

divided into

hominids, great

apes, and lesser

apes.

– Hominids include

living and extinct

humans.

12.3 Origin of Life

• Bipedal means walking on two legs.

– foraging

– carrying infants and food

– using tools

• Walking upright has

important adaptive

advantages.

12.3 Origin of Life

There are many fossils of extinct hominids.

• Most hominids are either the genus Australopithecus or

Homo.

• Australopithecines were a successful genus.

• The Homo genus first evolved 2.4 million years ago.

12.3 Origin of Life

Modern humans arose about 200,000 years ago.

• Homo sapiens fossils date to 200,000 years ago.

• Human evolution is influenced by a tool-based culture.

• There is a trend toward increased brain size in hominids.

Australopithecus

afarensis

Homo habilis Homo

neanderthalensis

Homo sapiens