Evolution__3rd_Edition

..
. . . partly by symbiosis
Atmospheric oxygen concentrations
increased over time
mitosis, in which an apparatus of mobile spindles is formed and pulls the duplicated
chromosomes apart. Eukaryotes also have meiosis. There are many other differences of
structure between the two kinds of cell (Figure 2.1, p. 22).
Mitochondria and chloroplasts almost certainly originated by symbiosis (Section
10.4.3, p. 265). Their symbiotic origin was first suggested by the morphological similarity
of the organelles to bacteria. The theory has since been strongly supported by
molecular evidence. The genes in the mitochondria of a eukaryotic cell are more similar
to genes in free-living bacteria than to comparable genes in the nucleus of the cell that
the mitochondria is living in (Gray et al. 1999).
The evolution of the nucleus and then mitosis and meiosis were probably separate
events, perhaps before a perhaps after a the origin of the organelles. The other differences
between eukaryotes and prokaryotes could have evolved at other times. The
origin of the eukaryotic cell would then have been a multistage process, extending over
a long time period.
An important event associated with the origin of eukaryotes is the evolution of
photosynthesis, or of photosynthesis on a mass scale. Photosynthesis itself probably
originated earlier a indeed Schopf’s 3.5 billion-year-old possible microbes may have
been photosynthetic a but around the time that eukaryotic cells were evolving there
was also an increase in the quantity of oxygen, suggesting that photosynthesis was
becoming much more important. Atmospheric oxygen would not have increased
immediately following the evolution of photosynthesis. The first oxygen would have
been absorbed by rocks, which became oxidized (indeed the oxidized form of ironcontaining
rocks is the main way that photosynthesis is inferred in the geological
record). Oxygen would have accumulated in the atmosphere only after the rocks had
absorbed all the oxygen they could.
A little over 2 billion years ago, atmospheric oxygen concentration probably
spurted up. The most likely reason is that photosynthesizing organisms had become
more abundant and were pouring out oxygen as a by-product. Also, the chloroplastcontaining
cells of eukaryotes were more efficient photosynthesizers than the prior
prokaryotes, and this is why the oxygen concentration increased at about the time
when eukaryotes were evolving. Whatever the reason, when oxygen was first released in
large amounts it was probably a poison to most existing forms of life, because they had
evolved in environments with little oxygen; there may have been an ecological disaster.
Subsequent forms of life have mainly descended from species that evolved to tolerate,
and then make use of, this chemical novelty. Aerobic respiration, using mitochondria,
may have become advantageous around this time.
18.3.3 The origin of multicellular life
CHAPTER 18 / The History of Life 533
“Multicellular” life is used to refer not simply to the presence of more than one cell in
an organism, but to more than one kind of cell a that is, to cell differentiation. Life
forms with more than one kind of cell have at least a rudimentary development. They
develop from a single-celled zygote to an adult with specialized cell types. The origin of
development is an important step in the evolution of life. Life forms consisting of rows
or mats made up of many identical cells had existed early in life. Schopf’s (1993) paper