Evolution__3rd_Edition

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Figure 18.3
Around 2–3 billion years ago,
prokaryotic life had evolved
to live in a variety of habitats,
using a variety of metabolisms.
The illustration shows
hydrothermal systems around
structures called komatiite
shields, chemotrophs in midocean
ridges, and various life
forms in coastal, lacustrine,
and oceanic waters. Redrawn,
by permission of the publisher,
from Nisbet (2000).
Early cellular life was prokaryotic
Hydrothermal
communities around
andesite volcanoes
Light
Cu, Mo, Zn, S
Stromatolites
Lake
communities
Light
year period (for instance, Knoll & Baghoorn 1977, and Schopf 1999 reviews the evidence).
Cells had therefore likely evolved by 3.5 billion years ago, or soon afterwards.
18.3.2 The origin of cells
Coastal sediment
S-microbial mats
Light
Organic
debris
S cycle and
methanogens
Planktonic
cyanobacteria
Open ocean
Light Light
More oxidized
More reduced
CHAPTER 18 / The History of Life 531
Hydrothermal
supply of metals
and reductant
in deeper water
Mid-ocean ridge
chemotrophic
community
Mg, SO 4
Fe, Mn, S,
CH 4, H 2
Hydrothermal
systems around
komatiite shields
Fossils of cellular prokaryotic life have been found at several sites, aged between 3.5 and
2 billion years ago. They often exist in the form of stromatolites. Stromatolites are
layered structures that are formed when cells grow on the sea surface, and sediments
are deposited among or above the cells. The cells then grow up to the light, leaving a
mineralized layer below them. As the process is repeated over time, a stromatolite
builds up, consisting of many mineralized layers. Stromatolites are still formed at
certain sites in the world today, but are rarer than in the past. One reason may be that
grazers now consume any cell mats as they form. In the past, grazers did not exist and
stromatolites could accumulate. Around 2–3 billion years ago, microbial prokaryotic
life seems to have existed in several environments, and to have evolved several
metabolic processes (Figure 18.3).
Thus, cellular life was flourishing by 2–3 billion years ago. However, the origin of
cells was probably not an evolutionarily inevitable step. Unadorned replicating molecular
systems could have persisted, the molecules being replicated as their component
building blocks bonded to them and formed copies, or near copies, of the whole. For
the system to become more complex, it needs enzymes and metabolic systems that
enable it to harvest resources more powerfully, or to exploit the resources better by
converting them into the molecular units needed for replication. This step is difficult
Lava
Light
Ni, Co, Fe,
S, Mg