How old is life on Earth?

Amid all the excitement surrounding the seven Earth-like planets discovered around the pint-sized star Trappist-1 in the constellation of Aquarius, it seems surprising that there’s scant mention of how old the stellar system is. The estimate is upward of 500 million years, and it’s worth noting because, if a paper just published in Nature is correct, that’s about as long as it took for life to get started after the Earth formed.

The new work, from an international team led by Matthew Dodd and Dominic Papineau of University College London, describes structures found in Canadian rocks that they interpret as fossils of microorganisms. If that’s so, it would be possibly the earliest evidence for life on the planet: the rocks are known to be between 3770 and 4280 million years old, while our planet itself is thought to have formed about 4600 million years ago.

Identifying very ancient microfossils is always contentious. All you have to go on are ambiguous squiggles and weird-shaped structures visible in the rock under a microscope, which may have a chemical composition suggesting—but rarely proving beyond doubt—that they were produced from living things. This study will be no exception, and the team’s conclusions are already being vigorously debated.

One key feature, though, is that these mineral formations include clusters of microscopic tubes, about a fiftieth of a millimetre wide, made from iron oxide. Similar iron-oxide tubes have been reported before in the minerals found at hydrothermal vents: chimney-like geological structures on the sea bed, from which hot water warmed by volcanic magma beneath the seafloor surface streams out.

There, the tubes are formed from bacteria that transform dissolved iron into the mineral. Hydrothermal vents often support diverse ecosystems, even though they are far too deep under the sea for sunlight to drive photosynthesis. Instead, the life is sustained by the heat and the rich brew of chemicals in the hot vent water.

It’s now widely believed that deep-sea vents may have been where life on Earth began. It is proving easier to construct plausible theories of how primitive cell-like systems, with membrane-like compartments that seal them off from their surroundings, could have arisen in those circumstances, than to see how carbon-based molecules floating about freely in waters at the ocean surface could have coalesced into the first cells. The rocks studied by Dodd and colleagues were indeed once part of the sea bed.

Microfossils 3700 million years old have been claimed previously in Greenland rocks, but not everyone considers those claims convincing. The oldest signs of life that are generally accepted date back to 3500 million years ago. If Dodd and colleagues are right, however, it’s not impossible that life existed on Earth more than 4200 million years ago. A few hundred million years might not seem a big deal in the grand scheme of things, but they certainly were on the early Earth. In its infancy, the planet had a very torrid environment. Riven with volcanism and bombarded with meteoritic debris not yet swept up from the formation of the planets, it seems a hellish place for life to take root—that’s why this aeon, until around 4000 million years, is called the Hadean. By 3500 million years ago, things had calmed down considerably.

It’s generally not even thought that there were oceans until about 3800 million years ago: the water was mostly vaporised. It’s hard to see, then, how life could have got going in such an environment—how, after all, can you have deep-sea hydrothermal vents if there were no seas?

But even if these microfossils—if that’s truly what they are—were formed towards the nearer end of the potential age range, this would still suggest that life started almost as soon as it was possible for it to do so. Could that really have then been a lucky accident, as some scientists have long thought? Rather, it would seem that life couldn’t wait to get going—that it was, perhaps, an inevitability. That’s indeed what some theories now imply. Given a source of energy (such as hydrothermal volcanism) and the right ingredients, such as the basic chemical compounds from which more complicated carbon-based molecules can be formed, the sort of complex self-organization that must have been manifested in even the most primitive of nascent life forms may cohere almost as a law of nature.

And that’s where the Trappist planets come in.

It might seem a little odd that seven more “extrasolar” planets (detected by NASA’s Spitzer Space Telescope and other Earth-based telescopes) should excite so many headlines anyway, given that we already knew of almost 4000 of these worlds around other stars. We have only just got over the excitement of Proxima b, an Earth-like planet seen around the nearest star to Earth, Proxima Centauri. What’s more, we already knew of three of the Trappist-1 planets back in May of last year.

Part of the excitement is that Trappist-1 too is, in galactic terms, a near-neighbour, only about 40 light years away. Those planning to get a spacecraft headed towards Proxima b might now be thinking of firing it in another direction and waiting a little longer, given the far greater wealth and diversity of riches this stellar system has.

But really, the glamour here comes mostly from that almost absurd abundance. It was always something of a thrill for planet-seekers to discover “Earth-like” worlds, but the term was sometimes used liberally to denote any planet that might conceivably have liquid water on its surface—orbiting the parent star, that is, within the so-called habitable zone, not so hot to be baked dry or so cold to be frozen solid.

That at least three (of the seven) planets in this single stellar system are genuinely Earth-like—of similar size and density, and all within the habitable zone—is, however, truly extraordinary. The evocative artists’ impressions of these worlds produced for NASA certainly play up a supposed resemblance, but there’s nothing overtly fanciful about those images. The planets denoted Trappist-1f and 1e, both lying comfortably inside the habitable zone, are shown with balmy-looking oceans under wispy clouds, surrounded by a shell of ice. All of these planets are thought to be “tidally locked,” meaning that they don’t spin on an axis like the Earth but are fixed in orbit with the same size always facing the star. That means the dark sides would be permanently cold.

We have to remember, though, that this solar system is very different from our own. Trappist-1 is a red dwarf star, much smaller and dimmer than our Sun and close in size to the planet Jupiter. Some of the planets are only in the habitable zone because they are so close to this relatively cool sun, and this means that they have very short orbital periods: their “years” last from between just one and a half to 20 terrestrial days. They would have no seasons to speak of.

What’s more, being this close to a red dwarf is a testing experience. These stars—which are in fact the most abundant type in the galaxy—are temperamental, especially in their first billion years or so of existence. They emit a lot of ultraviolet radiation, which is apt to split molecules apart (that’s why UV from sunlight can damage our genes) and can be harmful to life as we know it. The UV levels can become particularly hazardous during episodes of flaring to which these stars are prone, which also shower the planets in yet more damaging X-rays.

One key question, then, is whether the planets in the habitable zone have atmospheres like Earth’s, thick enough to absorb much of the UV rays and offer protection (on Earth much of that radiation is absorbed by the ozone layer). A study by two astronomers at Cornell University, Lisa Kaltenegger and Jack O’Malley-Jones, concludes that Earth-like atmospheres with enough oxygen to provide an ozone layer should render the surface relatively safe. But if the atmospheres are thinner, the prospects for life don’t look good unless Trappist-1 turns out to be a relatively quiescent star. In that case, the pair suggest, life might be able to survive underground or deep underwater (at hydrothermal vents!)—but if it does, the chances of our detecting its signature in the chemical composition of the atmosphere are much smaller. Another possibility, though, could be for Trappist organisms to evolve the ability to absorb UV rays and re-emit the energy at longer, safer wavelengths, in the visible part of the spectrum, as some “biofluorescent” corals do. And if they did that, we might even hope to detect the light they emit.

It’s tempting to imagine, once you see the artist’s impressions of these planets, that they must surely support life somehow. But the fierce outbursts of their parent star aren’t the only challenges. Some researchers believe that the conditions on Earth—with our moon-driven tides, our day-night cycles and our seasons due to the tilt of the rotation axis—are rather special and essential features for making the place habitable. On the other hand, if life began in deep ocean hydrothermal vents, far from sunlight and sloshing shorelines, it’s less clear whether those things mattered.

At any rate, the Trappist system is by far the most promising and exciting place so far discovered for searching for extraterrestrial life. Quite aside from its own virtues, it suggests that planetary environments at least somewhat like our own might be very common in our and other galaxies. At the same time, this family of planets highlights the fact that we’ll be able to do little more than speculate about life on other worlds unless we develop a better understanding of how, and how readily, it began on our own.