Life’s Complexity Began With Poop
- 10:48 AM
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Every now and then, science puts forth a theory that — at least on a bitterly cold December day, with a flu infection stirring fatigue in a certain science journalist — resonates with grand poetic truth. The theory: the incredible complexity of life on Earth, the myriad of forms and forms and functions, owes its existence to poop.
So suggested Australian geoscientist Graham Logan, one of many scientists to contemplate the mystery of Cambrian species radiation. For three billion years, life plodded along at a pace that makes glaciers look frantic; then, during the Cambrian, the major animal groups appeared. Clumps of single-celled bacteria and plankton suddenly gave way to creatures with shells, nervous systems, sensory organs and maybe even backbones.
The rapidity of speciation has perplexed generations of scientists, among them Darwin himself, who devoted a whole chapter to the Cambrian in On the Origin of Species and considered it a potential objection to his theory of evolution. Darwin eventually concluded that the Cambrian’s evolutionary divergences started earlier, but left no trace in the fossil record. Stephen Jay Gould called it "the most remarkable and puzzling event in the history of life."
What’s at the heart, then, of this most remarkable and puzzling event? Poop, says Logan.
At that point, Earth’s oceans were rich in carbon and poor in oxygen.
Photosynthesizing plankton produced some oxygen, but it was consumed by bacteria and offset by slow-sinking carbon. The arrival of multicellular organisms changed all that: they competed with bacteria for plankton, and their carbon-rich feces dropped quickly to the ocean floor. Bacteria dwindled; oxygen levels rose, especially in sunlit upper waters; and the ecological gates were thrown wide-open to the parade of animals that soon followed.
To support the theory, Logan noted an early-Cambrian drop of carbon 13
levels in bacteria-formed sedimentary deposits. Carbon 13 is an isotope that accumulates along the food chain; low levels suggest that bacteria became less plentiful, as would be expected if they were out-competed by newly multicellular organisms.
Logan’s is hardly the only explanation of the Cambrian cornucopia, but it’s certainly the most appealing. However, when I emailed him to ask about its popularity and — inasmuch as science is often valued for the metaphors and analogies it offers — its meaning, Logan responded,
"There is no deep meaning to the model. It is only a model. Personally
I love the ideas of models and concepts but once people get overly attached to ideas and concepts it can get a bit ugly. Hence, I’m quite happy to have developed an idea and was very happy to move on to other areas of work."
Alas, it seems the poop theory of life will remain unexplored. But perhaps that’s for the best; this way it can’t be disproven, and can keep offering solace on cold, flu-stricken, deadline-driven winter days.
The Poo Theory of Life [Environmental Graffiti]
Life’s grand explosions [Discover]
Images: Ernst Haeckel