Evolution__3rd_Edition

532 PART 5 / Macroevolution
Cells had advantages over noncellular
life
Eukaryotic cells evolved maybe
2 million years ago ...
for at least two reasons. One is mutational error, which becomes increasingly damaging
as the replicating molecule increases in size. The other is that any advantageous
innovation a for example, one that can produce useful molecules a will share its
produce with all the other competing replicating molecules in the locality. A “selfish”
replicating molecule, that used resources manufactured by others but did not itself
manufacture them, would have a selective advantage over other replicating molecules
that both manufactured and used resources (Section 11.2, p. 294).
This second difficulty was probably overcome by the evolution of cells. If the
replicating molecules are enclosed within cells, the products of their metabolism are
confined to the cell that produced them and are not available for any selfish replicating
molecules outside. Another advantage of cell membranes is that metabolic enzymes
can be arranged spatially; a chain of metabolic reactions can then operate in an efficient
sequence. Thus the first cells were probably little more than replicating molecules
either surrounded by, or arranged within, membranes. Modern prokaryotic cells are
complex versions of this form of life.
The deepest classificatory division of cellular life is a three-way divide into archaeans,
bacteria, and eukaryotes. Archaea and bacteria are both prokaryotes, and both existed
on Earth 2–3 billion years ago. The other kind of cell, the eukaryotic cell, evolved after
the prokaryotes. The time of origin of eukaryotes is uncertain. The oldest figure is
about 2.7 billion years ago. Brocks et al. (1999) found chemical fossils of certain fats
that are characteristic of eukaryotic metabolism in 2.7 billion-year-old Australian
rocks. This may mean that eukaryotes had evolved by then. Or it may mean that the
fats are not a good signature of eukaryotic life. After all, new prokaryotes, with an
ever-expanding range of metabolic skills, are being discovered every year. Thus, the
chemical fossils are not convincing evidence of eukaryotic origins, but they raise the
possibility that eukaryotes had already evolved 2.7 billion years ago.
The earliest fossil cells that have been proposed to be eukaryotic were found in an
abandoned mine in Michigan and are described by Han & Runnegar (1992). The fossils
are corkscrew shaped and closely resemble later algae (algae are eukaryotes). The main
criterion for distinguishing eukaryotic cells from prokaryotic cells in fossils is cell size.
Eukaryotic cells are typically larger than prokaryotic cells, though there is some overlap
in their size ranges. The Michigan corkscrews are huge a about 0.5 in (1 cm). If that is
a single cell, it is surely eukaryotic. Unfortunately, the fossilization process did not
preserve any signs of cell membranes. A skeptic can still suggest that the Michigan
corkscrews are really multicellular, in which case they could be prokaryotic. If we move
on to about 1.8 billion years ago, many fossil cells exist that are generally accepted to be
eukaryotic.
Molecular clock studies suggest the eukaryotes originated in the 2.2–1.8 billion years
ago range. Thus the body fossil and molecular evidence agree, but chemical fossil
evidence hints at an earlier date. Around 2 billion years ago is a generally quoted, if
uncertain, date for the origin of the eukaryotes.
The origin of the eukaryote cell could have been spread over many hundreds of
millions of years. Modern eukaryotes differ from prokaryotes in a long list of features.
The formally defining difference between eukaryotes and prokaryotes is the presence
or absence of a nucleus. Eukaryotes also possess organelles, including mitochondria
and (in plants) chloroplasts. Eukaryotes have a special process of cell division called
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