Why Oxygen Doesn’t Mark the Dawn of Complex Life

The advent of oxygen is supposed to mark the dawn of life as we know it. Accept that it doesn’t, and no one has ever quite understood why.

When oxygen levels on Earth rose, it took yet another hundred million years– a period longer than the entire history of the dinosaurs — before evidence of complex life first appeared — that’s sluggish even for evolution. Now, a new study reveals that levels of iron may have instead been responsible for this delay.

Complex life, everything from bread yeast to plants, bugs and humans, shares a distinct cell type that thrived out of oxygen’s unique chemistry. The secret? Our mitochondria, known as the oxygen-combusting powerhouse of our cells. Biology’s big breakthrough came from the discovery that mitochondria were once an independent, free-living bacterium that later evolved to live symbiotically inside the cell of another microbe. In other words, the cells of complex life — eukaryotic cells — formed from a fusion of different microbes.

But just what conditions drove these microbes together still remains one of biology’s biggest mysteries. Oxygen has long been the central focus, but the timelines simply don’t add up. Stumped by the evolutionary disconnect between evidence of oxygen and evidence of complex life, researchers have been looking at what else restrained life to Earth’s anoxic shadows.

Now, a new study reveals that a substantial drop in global iron levels coincides with right about when complex life first appears in the fossil record — some 1.6 billion years ago. Delving into ancient layers of rock, evolutionary biologist Zachary Adam says they’re “challenging oxygen-centric views”. Looking at geological iron records together with the ability of microbes to tolerate the metal, they looked at how levels of iron held back evolution.

Results from the new study suggest that when oxygen rose, it wasn’t a new opportunity, but rather a new problem for microbial life. Embedded within our Earth’s crust, banded-iron formations show that levels of iron in ancient oceans were thousands of times higher than those of today. Iron is essential in the chemistry of life, but when together with oxygen causes potent oxidation. Just look at any old iron nails left out to rust in the elements. Reactions like this inside a cell are lethal.

So, instead of springboarding evolution, oxygen’s rise initially created stressful and challenging times — even for simple, single-celled life. Such high iron levels would have “disrupted cellular systems and triggered biochemical stress”, says Adam, which likely held back the fusion of microbes from which complex — eukaryotic—cell types emerged.

Of course, scientists can’t rule out the possibility that complex life evolved much earlier, but its evidence is missing from the fossil record. In that case, stressful, iron-rich oceans likely still held evolution up, but some complex cells could have first emerged slowly in obscure low-iron refuges, like freshwater habitats, the study’s authors report.

Next steps in this research involve retracing the origins of iron and oxygen regulatory mechanisms within ancient, shared relatives of all complex life.

Understanding the conditions that allowed complex life to form on Earth can also help in our searches for life elsewhere in the universe. Oxygen and water are often used as indicators for potential life, but now it seems iron, too, ought to be included within the equation.

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