The Origin & History of Life

The Earth is about 4.5 billion years old. Evidence shows that life has existed for about 3.5 billion years, and that it has diversified over that time into the roughly 8 million species alive today. The oldest widely accepted fossils are stromatolites — layered structures built by cyanobacteria (photosynthetic prokaryotes) — found in rocks in Western Australia and dated at about 3.5 billion years old.

The fossil record is the main record of this history, but it is incomplete. Fossils form only where conditions favour preservation, so many organisms left no trace at all.

The timeline of life: prokaryotic cells appear about 3.5 billion years ago; the first eukaryotic cells do not appear until about 1.5 billion years ago.

CONDITIONS ON THE EARLY EARTH

The early atmosphere contained no oxygen and large amounts of carbon dioxide. Life as it exists today could not have survived under these conditions. Cells have therefore evolved from simple to more complex as conditions changed: once photosynthesis began releasing oxygen into the atmosphere, more complex, oxygen-using life — and greater diversification — became possible.

FOUR PROCESSES REQUIRED FOR LIFE TO BEGIN

Before the first cells could exist, four chemical processes had to occur:

1. Synthesis of simple organic molecules, such as amino acids.
2. Synthesis of larger organic polymers, such as proteins, from those simple molecules.
3. Formation of self-replicating molecules — RNA and DNA are the only known molecules that can copy themselves.
4. Packaging of these molecules into membranes, creating an internal environment different from the surroundings. Membranes may have formed spontaneously under early-Earth conditions.

The four processes required for life to begin, in order.

In summary: Simple organic molecules → organic polymers → self-replicating molecules → membrane-bound cells → prokaryotes → eukaryotes

THE RNA WORLD

The first simple cells may have used RNA as their genetic material, with DNA-controlled cells evolving later from these primitive RNA-controlled cells. RNA is a plausible first genetic molecule for four reasons:

1. RNA can be single-stranded or double-stranded.
2. RNA can store genetic information — some viruses still use RNA in this way.
3. RNA acts as a messenger and carrier, allowing proteins to be synthesised.
4. RNA is capable of self-replication, which is essential for passing genetic information to the next generation.

A ribozyme is an RNA molecule that acts as an enzyme: it folds into a shape that allows it to catalyse chemical reactions. Ribozymes catalyse the removal of introns after transcription and the synthesis of other RNA molecules. In modern ribosomes it is the rRNA component, not the protein component, that catalyses peptide-bond formation between amino acids.

COMMON ANCESTRY

The ancestors of all life forms lived over 3.5 billion years ago, and organisms are unified by common traits that reveal this shared ancestry. The more closely related two organisms are, the more recently their common ancestor lived — and the more traits they share, because less time has passed for evolution to make them different. The diversity of life is seen in morphology (body structure), physiology (body function), ecology (how organisms interact with their environment) and life history.

PROKARYOTES FIRST, THEN EUKARYOTES

A prokaryote is a cell with no membrane-bound nucleus or organelles; a eukaryote has both. Fossil evidence shows that prokaryotes existed before eukaryotes: cyanobacteria fossils are about 3.5 billion years old, while the first eukaryotic cells are thought to have formed about 1.5 billion years ago. This order is expected — eukaryotic cells are larger and more complex, and could not have survived the conditions of the Earth's early atmosphere.

ENDOSYMBIOSIS

The endosymbiotic theory ('endo' means within; 'symbiosis' means living in close association) proposes that mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by another cell by endocytosis. The engulfed cell was kept rather than digested, because the arrangement benefited both partners:

• Mitochondrion - a host that engulfed an aerobic bacterium gained aerobic respiration, and therefore a supply of energy.
• Chloroplast - a host that engulfed a cyanobacterium gained photosynthesis, and therefore a supply of organic molecules.
• The engulfed cell gained shelter and protection from predators.
• In separate events, infolding of the host's cell membrane formed the endoplasmic reticulum and the nucleus.

Endosymbiosis: a prokaryote engulfed by endocytosis becomes a double-membraned organelle. The inner membrane is the prokaryote's own membrane; the outer membrane comes from the host.

Structural evidence for the theory is found in every mitochondrion and chloroplast: