438 CHAPTER 17Precambrian Life
Our knowledge of the grand history of life, especially the origin, diversification,
and extinction of major groups of organisms, is derived from geological and pale-
ontological evidence and from phylogenetic studies of living organisms that have
helped us trace life’s history. We will frequently refer to the geological time scale
(see Table 17.1).
The Archean, prior to 2.5 Gya (2500 Mya), and the Proterozoic, from 2.5 Gya
to 541 Mya, are together referred to as Precambrian time. The oldest known rocks
formed in the presence of ocean water (3.8 Gy old) and contain carbon depos-
its that may indicate the existence of life. There is strong evidence of life by 3.4
Gya [12], and debated evidence as far back as 3.5 Gya, in the form of bacteria-
like microfossils and layered mounds (stromatolites; FIGURE 17.3) with the same
structure as those formed today along the edges of warm seas by cyanobacteria
(blue-green bacteria).
The early atmosphere had little oxygen, so the earliest organisms were anaero-
bic. When photosynthesis evolved in cyanobacteria and other bacteria, it introduced
oxygen into the atmosphere. Photosynthesis may have evolved as far back as 3.8 Gya,
but the first great increase in atmospheric oxygen was about 2.4 Gya, probably as
a result of geological processes that buried large quantities of organic matter and
prevented it from being oxidized [48, 57]. As oxygen built up in the atmosphere,
many organisms evolved the capacity for aerobic respiration, as well as mecha-
nisms to protect the cell against oxidation.
For about 2 Gy—more than half the history of life—the only life on Earth con-
sisted of two groups of prokaryotes, the Archaea and Bacteria, which are classified as
“empires,” or “domains.” The prokaryotes that descended from the LUCA diversi-
fied greatly in their metabolic capacities [19]. Photosynthetic, chemoautotrophic,
sulfate-reducing, methanogenic, and other forms soon evolved, and these forms
continue today to be the prime movers of the biogeochemical cycles on which ecosystems
depend. Today many archaea are anaerobic and inhabit extreme environments such
as hot springs. (One such species is the source of the DNA polymerase enzyme
[Taq polymerase] used for the polymerase chain reaction [PCR] that is the basis of
much of modern molecular biology and biotechnology.) The bacteria are extremely
diverse in their metabolic capacities, and many are photosynthetic. There was
extensive lateral transfer of genes among lineages during the early history of life
(see Figure 16.10) [30, 99]. The early phylogenetic history of prokaryotes was more
like a network than a simple branching tree, and it still is, to some extent.
A major event in the history of life was the origin of eukaryotes, which are distin-
Futuyma Kirkpatrick Sinauer Associatesguished by such features as a cytoskeleton and a nucleus with multiple linear Evolution, 4eTroutt Visual Services
Evolution4e_1703.ai Date 11-02-2016FIGURE 17.3 (A) Stromatolites (A) (B)
formed by living cyanobacte-
ria in Shark Bay, Australia. (B) A
3-billion-year-old stromatolite
from Western Australia has the
same structure as modern stro-
matolites. (A by D. J. Futuyma.)17_EVOL4E_CH17.indd 438 3/22/17 1:36 PM