New findings raise questions about when (and where) life began

In a seminal paper published last year in Nature Ecology and Evolution, Edward Moody and co-authors describe the deep-time genetic analysis that led them to push back the estimated age of the “Last Common Universal Ancestor” (LUCA) of all terrestrial life to sometime between 4.09 and 4.33 billion years old — several hundred million years older than previous estimates based on the fossil record alone. If the finding holds up, it would corroborate an earlier study by Elizabeth Bell, who claimed to have found carbon produced by life in a 4.1 billion-year-old mineral.  

Just a few years ago, most of the scientific community would have said life appeared on Earth only after the so-called “Late Heavy Bombardment,” the intense infall of asteroids and comets that occurred between 4.1 and 3.8 billion years ago (based on lunar impacts dated during the Apollo Moon missions). It was assumed this bombardment had completely sterilized Earth’s young surface so that life didn’t stand a chance until the pummeling ended. While scientists have recently questioned whether this heavy bombardment actually occurred — and to what extent it would have sterilized the early biosphere — most still believed that the first solid evidence of life on our planet is between 3.5 and 3.8 billion years old.

Life’s origin story

This new study might change that. While the exact dating of LUCA might seem unimportant to many readers — what’s a few hundred million years in the grand scheme of things? — the timing has far-reaching implications. If Moody and colleagues are correct, it means life arose extremely rapidly, almost as soon as our planet became habitable. 

According to our present knowledge, Earth formed about 4.56 billion years ago, and the Moon formed (violently) about 100 million years later. That leaves a very short interval, maybe just 200 million years or so, before the first living cells appeared. Moody’s team also found that this early life was already quite complex, encoding about 2,600 proteins, comparable to modern bacteria. It even had a primitive immune system that defended it from viruses.

Can the appearance of life really be that rapid, something like the inflationary phase of the Universe right after the Big Bang? That’s certainly faster than most of us previously thought. And if it happened that fast, shouldn’t it be relatively easy to decipher the steps life took to develop? 

Yet we still have no real handle on how all the functional components came together. We don’t even know for sure in which type of environment life originated. Maybe it was near “black smoker” hydrothermal vents, but it could also have been in ponds, tidal flats, or other locations. We just don’t know.

Panspermia

The breathtaking speed with which life appeared on Earth opens the door to another intriguing possibility known as “panspermia” — the idea that life originated on some other planet and arrived here inside meteorites. It’s an old idea, usually dismissed because it appears statistically very unlikely. I agree with that evaluation if the incoming meteorite came from outside our Solar System: Traveling through interstellar space for eons would likely sterilize any life forms due to harsh radiation. What’s more, any object arriving from so far away would be much more likely to fall into the Sun or Jupiter due to their much stronger gravitational pull.

But it’s a different story if a life-seeding meteorite came from Mars. It’s entirely plausible that life arose on the Red Planet independently. Our two worlds formed at about the same time, but Mars cooled much faster than Earth, and the geological record suggests that, shortly after its formation, the planet was habitable with plenty of water. Without a large moon to violently interrupt its early years, the life-starting window on Mars was actually longer than it was on Earth. And because Mars has lower gravity, rocks blown off its surface by asteroid impacts escape the planet more easily, to be hurled into the inner Solar System — toward Earth. The hundreds of Martian meteorites already discovered on Earth are proof of that.

Could primitive organisms have withstood the journey from Mars? The interior of the famous Martian meteorite ALH 84001 (in which David McKay and colleagues claimed to have found fossilized Martian life in the 1990s) was never heated above 40 degrees Celsius. Whether or not there really are fossils in that rock, as McKay claimed, organisms could theoretically have survived the trip to Earth. 

As long as we’re speculating, we might also consider another theory known as “directed panspermia.” More than 50 years ago, Nobel laureate Francis Crick (co-discoverer of DNA) and Leslie Orgel suggested that a highly advanced extraterrestrial civilization could have seeded Earth on purpose, exposing our planet to the first primitive cellular life, which, after gaining a foothold, would evolve to become more complex and even intelligent. While this may sound like science fiction, we probably could do it ourselves if we chose to. Just send dormant organisms inside a protected capsule to some habitable planet and open the hatch after arrival. (Whether this is ethical is another matter — for now, we’re just asking whether aliens might have done it.)

As intriguing as the panspermia hypothesis may be, indications still point to life getting its start right here on Earth, considering, for example, the similarity of Earth’s primitive oceans to the interior of microbial cells in terms of elemental abundances (cells are essentially bags filled with salt water!). In any case, if life did indeed begin somewhere else and arrive rather than arise on Earth, we still don’t know how it happened. Maybe studying Mars alongside our own planet will provide the clues we need to crack the case.