Geoscience Reference
In-Depth Information
cycle was quite active early in Earth history. Autotrophic metabolisms
also evolved early, and oxygen-utilizing genes expanded more recently.
According to this analysis, however, methanogenesis is a metabolism
that evolved later. Oops. I find this hard to believe, especially given the
geological evidence described above, but as I said, we are looking at the
initial application of this approach, not the last.
Let's try to pull all of this together. By all reckoning, Earth enjoyed
an active and diverse biosphere well before the evolution of oxygen-
producing cyanobacteria. This biosphere was fueled, mainly, by chemi-
cal compounds liberated during volcanism, underscoring again the im-
portance of plate tectonics in shaping life on our planet. The geologic
record provides support for an early diverse biosphere, as do new mo-
lecular approaches aimed at deciphering evolutionary history from
DNA sequences. It seems likely, though, that this early biosphere was
much less active than what we enjoy at present. In the next chapters
we will begin to look at the steps leading to the evolution of oxygen-
producing organisms and will begin to understand how the present bio-
sphere was formed.
