Geoscience Reference
In-Depth Information
is a beautiful piece of geochemical sleuthing and shows that methane-
producing organisms were also part of the ancient microbial ecosystem
at North Pole, Australia.
All in all, geological evidence suggests that many of the processes
that we have imagined were part of the early biosphere that was in place
3.5 billion years ago. These processes include methanogenesis, sulfate
reduction, and decomposition of dead organic biomass, which was likely
aided by a host of different fermenting bacteria. Unfortunately, indica-
tors of anoxygenic photosynthesis in rocks so old are not terribly robust,
so the geologic record is rather silent on the antiquity of this process.
We will, however, explore other ways of looking at the antiquity of an-
oxygenic photosynthesis in the next chapter.
There may, however, be another way to investigate the early history of
microbial evolution independently of the geologic record. The premise
is simple. All organisms on Earth contain a record of their evolutionary
history in their DNA. This is because the DNA of any organism, includ-
ing us, is the product of all of the changes that have occurred in its lin-
eage before the present time. The history recorded in DNA, however, is
complicated. It is influenced by a number of factors, including the du-
plication of genes, the birth of new genes, the loss of genes, the transfer
of genes between organisms, and all the mutations accumulated in the
DNA sequences through time. In principle though, we can compare the
DNA of one organism to that of another to understand how the DNA
differs between them, and if we compare enough different organisms,
we can understand the history of DNA evolution. Eric Alm from the
Massachusetts Institute of Technology (MIT) and his student Law-
rence David have taken a particularly sophisticated look at this issue.
They analyzed nearly 4000 genes from over 100 organisms and by doing
so, they offer a history of gene evolution. The results are spellbinding.
I have reproduced some of these results in
plate 5.
hat we have is a
history of the importance of different metabolic types through Earth's
history. There are many assumptions behind the construction of such
an evolutionary history, and this is also among the first attempts at this
fascinating approach. With all of this in mind, I stare at the results. This
data is, in principle, exactly what we are after. hat type of organisms
deined the biosphere 3.5 billion years ago? hen did sulfate reduction
evolve? Methanogenesis? The results in
plate 5
suggest that the sulfur
