The Origins of Life

• The Big Bang occurred about 18 billion years ago, creating the universe.

• The Earth formed about 4.6 billion years ago.

• Life on Earth probably originated between 3.5 and 4 billion years ago.

Evidence of First lifeStromatolites

• Stromatolites are mounds of banded sediment formed by mobile cyanobacteria.

• They are found in the warm waters of Australia and off the coast of Baja, Mexico.

Evidence of First LifeStromatolites

• Stromatolites have been found that date back to 3.5 billion years ago.

• They are strikingly similar to those produced by modern day bacteria.

Evidence of First LifeFossils

• Fossilized filamentous bacteria has also been found that dates back to 3.4 billion years.

• This fossilized prokaryote was found in a 3.4 billion year old stromatolite in Western Australia.

Early Earth

• When the Earth first formed, it was very different from its current state.

• It was filled with reducing gases, like H2, NH3, CH4, CO, and CO2.

• Most notably, the atmosphere of early Earth was completely anaerobic.

• It lacked a protective ozone layer.

Chemical Origins

• In the 1920’s, Oparin and Haldane predicted that the conditions of early Earth favored the spontaneous formation of organic compounds.

• In 1953, Miller and Urey tested this hypothesis.

From Molecules to Cells

1. Small organic monomers formed in reducing atmosphere.

2. Monomers joined to form polymers – binding sites in clay helped concentrate the monomers, while metal ions acted as catalysts.

3. Polymers aggregated to form protobionts.4. A molecule of heredity originated.

Protobionts

• Protobionts are aggregates of molecules that have some cell-like qualities such as:– Self-assembly– Response to osmotic

forces– Ability to store a

membrane potential– Engulfing and splitting

First Life

• Prokaryotic• RNA as genetic material• Heterotrophic – absorbed organic molecules from

surroundings• Anaerobic – oxygen was not present in

atmosphere until it was put there by photosynthetic bacteria as a waste product (began accumulating about 2 billion years ago

Origin of photosynthesis

• Some time after the first living things appeared, cells developed that could perform photosynthesis.

• These organisms put oxygen into the air as a waste product.

• About 1.5 billion years ago, oxygen still only made up about 1% of the atmosphere

• Accumulation of oxygen in the atmosphere is thought to have ended chemical evolution

Open for debate

• Some scientists believe organic molecules came to Earth from outer space. Meteors have been found containing amino acids and nucleic acids (including several amino acids nonexistent on Earth).

• There may have been a simpler hereditary molecule that preceded RNA, or maybe RNA was never genetic material.

• Maybe life began near hydrothermal vents (and still forms there?)

Taxonomy – the classification of living things

• From that first cell, life has diversified into millions of different forms.

• These forms are divided into five major kingdoms: Animalia, Plantae, Fungi, Protista, and Monera.