Page 208 - Introduction to Paleobiology and The Fossil Record

THE ORIGIN OF LIFE 195

(a)

(b)

(c)

Figure 8.8 Prokaryote fossils from the Gunflint Chert of Ontario, Canada (c. 1.9 Ga): (a) Eosphaera,
(b) Kakabekia, and (c) Gunfl intia. Specimens are 0.5–10 μm in diameter. (Redrawn from photographs
in Barghoorn & Taylor 1965.)

least 2.7 Ga, but it is also the oldest hint of transfer, fl agella for movement and chloro-
the occurrence of eukaryotes, long before any plasts in plants for photosynthesis. There are
fossils of that major life domain. also many major biochemical differences
between prokaryotes and eukaryotes.
The origin of eukaryotes is mysterious
LIFE DIVERSIFIES: EUKARYOTES because they are in many ways so different
from prokaryotes. The most attractive idea
Eukaryote characters
for their origin is the endosymbiotic theory,
Evidence about the earliest evolution of the proposed by Lynn Margulis in the 1970s.
three domains is scant. It has long been According to this hypothesis (Fig. 8.9c), a
assumed that prokaryotes (i.e. Archaea and prokaryote consumed, or was invaded by,
Bacteria) were the sole life forms for a billion some smaller energy-producing prokaryotes,
years or more, and that eukaryotes came and the two species evolved to live together
much later. This evidence is much more in a mutually benefi cial way. The small invader
blurred now (Embley & Martin 2006), and was protected by its large host, and the larger
the fossils, biomarkers and molecular evi- organism received supplies of sugars. These
dence suggest that eukaryotes might be as old invaders became the mitochondria of modern
as one or other of the prokaryote domains. eukaryote cells. Other invaders may have
The appearance of eukaryotes was important, included worm-like swimming prokaryotes
whenever it happened, because they are (spirochaetes) that became motile fl agella,
complex and include truly multicellular and and photosynthesizing prokaryotes that
large organisms. became the chloroplasts of plants.
Eukaryotes are distinguished from pro- The endosymbiotic model is immensely
karyotes (Fig. 8.9a, b) by having a nucleus attractive, and some aspects have been con-

containing their DNA in chromosomes (pro- firmed spectacularly. Most notable is that the
karyotes have no nucleus, and they have only mitochondria and chloroplasts in modern
a circular strand of DNA) and cell organelles, eukaryotes are confi rmed as prokaryotes,
that is, specialized structures that perform key the mitochondria being closely related to
functions, such as mitochondria for energy α-proteobacteria and the chloroplasts to