Charles Darwin, father of evolutionary biology, wrote in 1871 that life first emerged in “warm little ponds”,
which he imagined to be small wells of water and chemicals, heated by
the sun and surrounded by rocks and air. With these few ingredients and a
big dose of randomness, he posited, the basic elements of life clicked
together, leading to simple life forms, like bacteria. Their evolution
over millions of years eventually led to the sophisticated life forms
that now inhabit the planet.
These days, scientists generally agree with the idea that the
original recipe for life was pretty simple, but they’re not sure what
ingredients were necessary for those early life forms to make the leap
into complex forms of life, like animals. Many scientists theorize that, since all complex life — involving cells that have multiple components
— now relies on oxygen to breathe, it must have happened at a time when
there was plenty of oxygen in the air. But the scientists behind a 2018
study published in Nature report that oxygen in the atmosphere didn’t rise to significant levels until after complex life arose — suggesting that oxygen wasn’t all that important after all.
“This is significant because it provides new evidence that the
origination of early animals, which required O2 for their metabolisms,
may have gone on in a world with an atmosphere that had relatively low
oxygen levels compared to today,” said study co-author Daniel Stolper,
Ph.D., an assistant professor of Earth and planetary science at the
University of California, Berkeley, in a statement.
In previous studies, scientists determined that complex life first emerged around 700 and 800 million years ago,
sometime between huge ice ages. The history of oxygen on Earth,
meanwhile, is a bit cloudier. Scientists believe there was no oxygen for
Earth’s first two billion years, and then, some 2.3 to 2.5 billion
years ago, a little bit of oxygen showed up (they can tell because it
turned some rocks red with rust-like compounds). But deposits of
fossilized charcoal have shown that it wasn’t until at least 400 million
years ago that there was enough oxygen in the atmosphere for forest
fires to burn. That leaves a 2.1-billion-year period during which there
was minimal oxygen — but, strangely, still evidence of life.
At one point in those 2.1 billion years, the geochemists show,
the amount of oxygen in the air reached a concentration high enough that
it led to the deep sea becoming oxygenated — sometime between 540 and
420 million years ago. They came to this conclusion by looking at rocks
formed by undersea volcanoes — in particular, the iron inside them.
Anyone who has encountered rust on a car knows that oxygen and
iron react in very obvious ways, and the reaction is no different in the
underwater rocks. Seawater flowed through them as they first formed, so
the iron in the rocks carries the chemical signature of the water. It
soon became clear, from the oxidation of the iron, when the sea became
full of oxygen.
More importantly, it also became clear that complex life had
existed on the Earth long before the oxygenation of the sea took place.
This finding complicates matters for researchers trying to
figure out when complex life on Earth emerged, especially those who
believe that, since all life breathes oxygen, the event was inextricably
tied to the oxygenation of the atmosphere. Since that now doesn’t seem to be the case, scientists must think on different theories, like one posited by scientists in Nature
in 2017, suggesting that the explosion of complex life coincided not
with a rise in oxygen but with the first big boom in algae growth.
The more we learn about the origins of life, both simple and
sophisticated, the more puzzling life seems to be. In some ways, it
doesn’t seem to be very complex at all. For the most part, scientists
pondering the mystery of life’s origins still think along the same lines
as Darwin, proposing that the original recipe really wasn’t very
complicated at all — and perhaps could have been cooked up someplace
other than Earth. Paul Niles, for one, a planetary geologist with NASA
investigating the possibility that life could have emerged on Mars, said in a statement in October 2017 that sometimes, life “doesn’t need a nice atmosphere or temperate surface, but just rocks, heat, and water.”
Since abundant oxygen now doesn’t seem to be all that necessary for simple life to develop into something more, there’s now an even greater possibility that complexity exists somewhere other than here.
Yasmin Tayag is a writer and former biologist living in New
York. A Toronto girl at heart, her writing also appears in The Last
Magazine and SciArt in America. You might recognize her as a past host
of Scientific American's YouTube series.
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