What enabled animal life to get more complex and diverse during the Cambrian explosion?

Jordana Cepelewicz writes:

When Emma Hammarlund of Lund University in Sweden first reached out to her colleague Sven Påhlman for help with her research, he was skeptical he’d have much insight to offer. He was a tumor biologist, after all, and she was a geobiologist, someone who studied the interplay between living organisms and their environment. Påhlman didn’t see how his work could possibly inform her search for answers about the rapid proliferation and diversification of animal life that, half a billion years ago, forever changed Earth’s evolutionary landscape.

In spite of Påhlman’s initial reservations, however, the pair has collaborated over the past four years to put forth a new interdisciplinary hypothesis, published in Nature Ecology & Evolution earlier this year, explaining why it took so long for animals to burst onto the scene.

For most of its 4.5-billion-year history, Earth has sustained life — but that life was largely limited to microbial organisms: bacteria, plankton, algae. Not until about 540 million years ago did larger, more complex species begin to dominate the oceans, but within just a few tens of millions of years (a blip on the evolutionary timescale), the planet had filled up with all kinds of animals. The fossil record from that period shows the beginnings of almost all modern animal lineages: animals with shells and animals with spines, animals that swam and animals that burrowed, animals that could hunt and animals that could defend themselves from predators.

Like many biologists, Hammarlund wondered why it took so long for complex animals to emerge — and why, when they finally did, it happened so suddenly. One of the leading theories about this hotly debated question holds that a skyrocketing rise in atmospheric oxygen around that time triggered what’s known as the Cambrian explosion. Earlier, when oxygen was scarce, the simple animals in the seas had anaerobic metabolisms that did not depend on it, and they even found oxygen problematic if not toxic. By shifting to aerobic respiration, however, animals gained an enormous metabolic advantage because the amount of energy that cells could produce per respiration cycle increased nearly twentyfold. That extra energy may have been what powered the greater complexity witnessed during the Cambrian period: increased biomass, improvements in their cellular systems, more complex body structures, and the capacity for energy-intensive movement and predation. [Continue reading…]

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