There is a great article on the dawn of the animals in the July 2009 edition of the New Scientist discussing why it is that we don't see many fossils before the Cambrian explosion, but a staggering variety afterwords.
The main point is that between 2.5 billion to 900 million years ago oxygen was only available in the atmosphere or in the top few meters of the ocean. The majority of life consisted of anoxic (non-oxygen using) bacteria. These circumstances constricted eukaryotic life to soft-bodied, sponge-like forms since anything thicker would have trouble getting oxygen. Furthermore, because of the difference in oxygen levels between the air and the sea, oxidative weathering on land would pour sulphur into the oceans. Bacteria would then turn this sulphur into hydrogen sulphide, a deadly gas for eukaryotes. This all changed somewhere between 900 million to 500 million years ago when ice ages stopped the delivery of sulphur to the oceans, allowing eukaryotic life a chance to flourish.
This is all pre-amble to what I personally think is the most salient point. That these super ice ages, so critical to the establishment of eukaryotic (and multicellular life), was triggered by the very beginnings of that life in the first place. This article and a book (that I haven't read but am going to have to pick up) called Darwin's Lost World, suggest that these ice ages were triggered by carbon dioxide being sucked out of the atmosphere by nascent animal life.
If this is true, it casts a whole new light on how life impacts the earth and vice-versa. Early animals changed the entire chemical composition, as well as the atmosphere and climate of our planet in order to further their own proliferation. For most of recorded history, either through religion, folk-tales or superstitions we always assume we are at the mercy of the elements. If this is the case, then it is actually the elements that are at the mercy of life.
Sort of puts a whole new angle on the climate change debate...
To reinforce your point about the impact of life on geology, I just recently caught part of the show on the early Earth (it was on the History channel!) - and was taken aback by the scale of oxidation of the Earths oceans.
ReplyDeleteApparantly the early ocean would have looked green to us since there was so much dissolved iron in the water. It was the emergence of oxygen generating stromatolites along the coasts of the emerging continents that oxidised this iron to "rust", which precipitated to the sea floor. This massive deposition of iron oxide is the source of most of the modern world's iron ore - and the rest, as they say, is history.
As you point out - we tend to assume that life is some fragile veneer on the surface of the planet, and underestimate the extent to which the scale of biological activity becomes planet changing.