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Until recently, many discounted the idea that life could have existed on Earth before 3.8 billion years ago because it was thought that heavy pummelling from asteroids would have made this impossible. But several lines of evidence are pointing to an earlier origin of life, and as we begin to question whether the late heavy bombardment really happened at all, it鈥檚 beginning to look like life started surprisingly early in our planet鈥檚 history.
The earliest fossil evidence 鈥� around 3.5 billion years ago
Although the fossil record is by nature patchy, fossils offer the most direct evidence of life鈥檚 ancient past. When the fossils in question are of microscopic, single cells, it can be hard to know for sure if they really represent preserved microorganisms or instead are weird geological shapes produced through non-living processes. But in Pilbara, Western Australia, there is decent fossil evidence of life around 3.4 billion to 3.5 billion years ago.
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Pilbara is special because it contains some of the oldest rocks on Earth. There鈥檚 a scarcity of deeply ancient rocks on our planet because, over time, rocks tend to get dramatically reshaped and reformed by Earth鈥檚 violent geological processes. But Pilbara is home to layered rocks dating back 3.48 billion years that appear to be ancient stromatolites 鈥� stacks of thin, fossilised bacteria layered on top of each other.
For a while it was unclear if these rocks simply looked like stromatolites by coincidence or if they truly had a biological origin, but a 2019 study found chemical signs that minerals in these structures have an organic origin. Earlier studies had found microfossils in Pilbara that look a lot like bacteria. These early organisms appear to have cell walls 鈥� a feature that bacterial cells have today.
And now there is further evidence of life around 3.4 billion to 3.5 billion years ago, thanks to a reinterpretation of strange shapes that have been found in a few places around the world 鈥� including Western Australia, but also Buck Reef in South Africa and others 鈥� and have been hard to explain.
These structures look biological but aren鈥檛 what you鈥檇 expect from bacteria 鈥� they鈥檙e too large and seem to have complex internal organisation within their cells. But as 快猫短视频 reported in July, it now seems that these closely resemble modern bacteria that have been stripped of their cell walls.
The earliest chemical evidence 鈥� around 4.1 billion years ago
Another approach to look for the earliest signs of life is to search for chemical signatures of biological processes. One of the most interesting lines of enquiry has been the analysis of zircon crystals from roughly 4 billion years ago, which can act as time capsules. One study of more than 10,000 zircons found tiny flecks of graphite 鈥� known as inclusions 鈥� dating to 4.1 billion years ago (see picture above). The exciting thing about these are their isotopes. Different isotopes of carbon occur naturally, but life on Earth preferentially absorbs and uses carbon-12 鈥� and the graphite flecks had a relatively high ratio of carbon-12 to carbon-13. If carbon-12 is proportionally more common in a sample than you would expect by random chance, it鈥檚 a clue that something alive has been involved.
These graphite flecks trapped inside zircon aren鈥檛 indisputable evidence that some form of biological processes must have been taking place 4.1 billion years ago, but they鈥檙e a tantalising hint. What鈥檚 more, when the finding was first published in 2015, it raised the prospect that we might have been wrong about not only the origins of life on Earth but also the story of the formation of our solar system.
The problem was the late heavy bombardment. This was a period from around 4 billion to 3.8 billion years ago in which we thought asteroids were pummelling our solar system鈥檚 innermost planets. Our vision of Earth at this time was a fiery, volcano-covered world that suffered constant collisions that would have made it impossible for life to have kicked off and survived during this time.
However, in recent years, the suggestion that the late heavy bombardment never happened has looked increasingly likely. The idea that early Earth underwent a particularly intense asteroid assault around 4 billion years ago largely came from our interpretation of our moon鈥檚 many craters, but over the past decade or so, analyses of moon rocks collected by the Apollo missions, as well as studying meteorites from Mars, have painted a picture of an early solar system without this especially heavy phase of bombardment.
The earliest genetic evidence 鈥� around 4.2 billion years ago
Abandoning the idea of a late heavy bombardment makes it possible to imagine that life could have begun only a few hundred million years after our 4.5-billion-year-old Earth formed. This is a dramatic change in how we view life on our planet 鈥� it no longer seems a given that Earth was uninhabited for around the first billion years of its existence.
Adding to this rethink is an improved understanding of what young Earth was like. It now looks like our planet may have cooled and gained oceans that could potentially have supported life by about 4.2 billion years ago, as Michael Marshall wrote about for 快猫短视频 in 2023.
That lines up quite nicely with some genetic evidence, a third way to investigate the origins of life on Earth. Analysing DNA in living organisms and inferring how much it has changed can be a very powerful tool for understanding the origins and evolution of life on Earth. But it always comes with caveats 鈥� we can鈥檛 know for sure that the assumptions made by such studies are accurate, and we can鈥檛 always be confident in the timing estimates these techniques produce.
However, these estimates can be improved by taking fossils into account, and an interesting 2018 study used gene sequences to build a family tree of life on Earth, and then used fossils to add timings to key shifts and splits that occurred along this tree. In this way, the study estimated that a monumental lifeform known as LUCA lived at least 3.9 billion years ago.
LUCA stands for 鈥渓ast universal common ancestor鈥�, and by this we mean the last living organism from which all life on Earth today is descended. Early life was almost certainly an ecosystem of different beings, but one of these organisms went on to become the genetic ancestor of everything that has lived since. We know this happened because all life today, no matter how diverse, has enough genetic similarity to show that they are distantly related.
And now, a new study that looked at many more genes than some previous analyses, has suggested that LUCA existed 4.2 billion years ago. As Michael Le Page reported for 快猫短视频 in July, this earlier estimate is partly because, unlike many other studies, the team behind it didn鈥檛 assume that LUCA could only have existed after the late heavy bombardment.
Rethinking life鈥檚 origins on our planet
Taken together, a different picture of our planet鈥檚 early years is coming into view. We used to think Earth was devoid of life for the first fifth of its existence. To know now that it wasn鈥檛 sterile for its first billion years offers a different perspective 鈥� that life is capable of emerging on a young planet only a few hundred million years old.
