Page 196 - Introduction to Paleobiology and The Fossil Record

Chapter 8

The origin of life

Key points

• Life originated by fusion of organic molecules in the first billion years after the forma-
tion of the Earth.
• The precursor to living cells may have been self-replicating RNA; a time before life
originated termed “RNA world”.
• Photosynthesis by a group of bacteria, called cyanobacteria, generated molecular oxygen
(O 2 ), and the atmosphere became oxygenated at a low level 2.4 Ga. Later, oxygen levels
increased further, around 0.8–0.6 Ga.
• The universal tree of life, reconstructed from gene sequencing of modern organisms,
shows there are three great domains: Bacteria, Archaea and Eucarya. The fi rst two are
prokaryotes, the last eukaryotes.
• The earliest fossils are bacteria in rocks up to 3.2 Ga, indicated by stromatolites, struc-
tures built by alternating algal mats and sediment layers.
• Cellular fossils 3.5 Ga are highly controversial; the first widely accepted cellular fossils

date from 2.5 Ga.
• Biomarkers, notably lipids, provide evidence for cyanobacteria and eukaryotes
2.7 Ga.
• The oldest eukaryotes, cells with a nucleus and organelles, date back perhaps 1.9 Ga.
• Red algae from 1.2 Ga show that sex had originated – they show mitosis, but also
meiosis, which is unique to sexual reproduction.
• Together with sex came multicellularity, the possession of many, often specialized, cells,

first seen in 1.2 Ga red algae.

Life is improbable, and it may be unique to this planet, but nevertheless it did begin
and it is thus our task to discover how the miracle happened.
Euan Nisbet (1987) The Young Earth