As the Boring Billion closes, things really happen. Geochemical evidence shows that we begin losing sulfidic waters about 800 million years ago. At the same time, paleontology tells us that eukaryotes are diversifying and expanding over large areas of the ocean. Molecular evidence suggests that animals start to differentiate around then.
There are now major fluctuations in the carbon cycle never before seen. We have multiple glaciations, of which at least two seem to have pretty much covered the world. And we have oxygen rise, so that we come to have a world much more like the world that we know. The 300 million years after the end of the Boring Billion are probably the most eventful 300 million years in our planet’s history.
So you would say that calling it the Boring Billion sells this period short?
Very much so. One reason is that understanding the interval’s stability may be more of a challenge than understanding the change we see both before and after. And we know it wasn’t that evolution stopped. In fact, there’s reason to believe that all of the properties of cell biology that made complex life possible in the next geologic era were put in place here: cytoskeletons that allow eukaryotic cells to change shape, and cell polarity that allows cells to send a molecular message to one side of the cell but not the other, and to interact with nearby cells. The molecular circuitry and cross talk that allow complex organisms like us to exist today all took root in the so-called Boring Billion.
You recently showed that the oceans had an abundance of
sulfide and a dearth of oxygen during a later period as well, around 500 million years ago. Evolution slowed down then, too. Is this a regular pattern?
Yes, but it is less and less frequent. If you look at the last 65 million years, in the so-called Cenozoic era, I don’t think there are any examples of globally widespread subsurface oxygen depletion. In the previous era, the Mesozoic, from 65 to 250 million years ago, there were six or seven such oceanic anoxic events. They were short, sharp shocks. Going back even farther, in the Proterozoic, these kinds of environments were everywhere. Over the course of time, it goes from being ubiquitous to repetitive to rare to absent—more evidence that we live at an unusual time in the history of Earth.
You are a member of the Mars Rover science team. What parallels do you see between the geologic history of Earth and Mars?
We can apply what we’ve learned about studying ancient rocks on Earth to Mars. NASA’s Mars Exploration Rovers have enabled us to examine 3.5- to 4-billion-year-old sedimentary rocks on Mars, in much the same way that we study ancient strata on Earth. We’ve learned that liquid water was present on the Martian surface during this interval, but also that its chemical makeup and short duration would have challenged any known life-forms. Mars was wetter very early in its history, but the probability that it was ever a blue planet like the Earth is, I think, remote. The more we learn about Mars, the more it seems to me a planet that’s very different from Earth.
So you don’t think we’ll find signs of life on Mars?
It’s not impossible, but I wouldn’t bet large sums of money on it.