Precambrian Biology - Origins of Life on Earth

fossil evidence

Carbon isotope ratios in 3.8 b.y. rocks (Isua Formation) from Greenland had been suggested to have resulted from organic activity. However a recent (2002) reinterpretation of the rock rock formation concludes otherwise.

The oldest fossils are 3.5 and 3.4 billion year old single-cell, photosynthetic, prokaryotic, filamentous cyanobacteria (blue-green algae) found in Western Australia and South Africa.

The oldest certain stromatolites (mounds of cyanobacterial mats) found so far are about 3.2 b.y., though the above-mentioned 3.5 b.y. microfossils were associated with stromatolite-like structures.

Prokaryotes are all single-celled organisms that lack membrane-enclosed nucleus and organelles. They reproduce by simple cell division creating duplicates of the parent; genes are not exchanged/combined from separate parents. Prokaryotes include the kingdoms Archaebacteria and Eubacteria.

Eukaryotes are organisms with membrane-enclosed nucleus and organelles (mitochondria, chloroplasts, golgi apparatus, endoplasmic reticulum, etc.). They probably evolved by about 1.6 b.y. The oldest fossil eukaryotes are acritarchs. These fossils appear very similar to the cyst or resting stage of modern dinoflagelates which are true algae. Eukaryotes include the kingdoms protista (protozoans and algae), fungi, plants, and animals.

(see The Six Kingdoms of Life)

Eukaryotes are now widely believed to have evolved by endosymbiosis between two or more prokaryotes. Evidence for this is the fact that mitochondria and chloroplasts both contain their own DNA and RNA which strongly suggests that they were once free-living organisms that came to live inside some host cell.

Sexual reproduction evolved among eukaryotes probably by about 1.1 b.y. Combining genes from 2 parents greatly increased the variations in populations greatly increasing the rate of evolution.

The first clear evidence of metazoans (complex multi-cellular animals) appeared in the rock record by about 630 - 600 million years ago in the Vendian Period of the Late Proterozoic as the soft body imprints of the Ediacaran fauna including probable members of primitive animal phyla porifera (sponges), cnidaria (jelly fish), and annelida (segmented worms). A recent (2002) paper surprisingly puports to find Ediacaran metazoans in strata that they date at 1 b.y. or so.

summary of important events in the fossil record of early life

3.5 b.y.

single-cell, filamentous cyanobacteria

prokaryotes (no nucleus)

3.2 b.y.

stromatolites (cyanobacterial mats)

earliest large fossils

1.7 b.y.

four-cell and colonial cyanobacteria

oxygen in atmosphere

1.6 b.y.

acritarchs = true algae

eukaryotes (cell nucleus)

?1.1 b.y.

sexual reproduction

increasing diversity

630 Ma

Ediacaran soft bodied fossils

metazoans (complex animals)

543 Ma

Cambrian Revolution

hard-shelled animals

relevant laboratory experiments attempting to simulate origin of life:
- Stanley Miller (1953): produced amino acids by simulating early Earth conditions
- Sidney Fox (1950s): polymerized amino acids into proteins in another early Earth scenario
- David Deamer: proteinoids: pseudo-living cells that divide and bud
- others have polymerized sugars and synthesized the basic building blocks of DNA and RNA
- it is still a vast leap from these compounds to actually assembling RNA and DNA into a reproducible genetic code

other considerations
- organic compounds such as amino acids appear to be common in the universe: meteorites, comets, and the interstellar dust contain them
- Martian meteorites contain cryptic evidence of possible life, though most now believe the observed features are probably of inorganic origin
- food chains at vent communities at midocean ridges are based on primitive chemosynthetic bacteria that could represent the oldest life on Earth, or in any case must be an evolutionary step earlier than photosynthetic organisms like cyanobacteria