As we know, the ‘tree of life’ is a vast, many-branched, tree of living organisms that all owe their existence to previous ancestors. Each twig on the tree is linked back to a larger branch – a common ancestor.
For instance, primates alive today, including humans, branched off from a common ancestor millions of years ago. The further back in time we go, the thicker the branches of common ancestry go. Even a gigantic class, such as mammals, once had a common ancestor, in this case, the cynodonts, who branched off from the more reptilian synapsids, themselves a group within the tetrapods – literally ‘four-limbed’ animals, including all reptiles, amphibians and birds – nearly 300 million years ago.
Tracing the stout branches of common ancestry even further, we logically arrive at the ‘tap root’: the common ancestor of all living things on Earth today. One clue to this unimaginably ancient, primitive ancestor is the DNA inside all living organisms.
The DNA inside all organisms alive today, from E coli to blue whales, has many similarities, which suggests it can all be traced back billions of years to a last universal common ancestor – LUCA. Many efforts have been made to understand LUCA, but now a study taking a broader approach has turned up some surprising results.
One surprise is that the common ancestor of all life on Earth may have evolved much earlier than previously thought, at the very dawn of the Earth itself, just a few hundred million years after the planet formed. Its genome may also have been much more sophisticated than previously thought.
“What we’ve been trying to do is bring people representative of different disciplines together to come up with a holistic understanding of when LUCA existed and what its biology was,” says Philip Donoghue at the University of Bristol in the UK […]
The researchers estimate that 2600 protein-coding genes can be traced back to LUCA, whereas some previous estimates are as low as 80. The team also concludes that LUCA lived around 4.2 billion years ago – much earlier than other estimates, and surprisingly close to the formation of Earth 4.5 billion years ago. “It suggests evolving life may be simpler than people have argued in the past because it occurred so early,” says Donoghue.
The researchers also argue against the ‘late heavy bombardment’ precluding the rising of life any time earlier. The LHB was a period about 3.8 billion years ago, when the Earth was pelted with masses of space debris. It has long been assumed that the constant cataclysms would have wiped out any nascent life.
Because their reconstruction suggests that LUCA had genes for protecting against UV damage, it is most likely that it lived at the surface of the ocean, the researchers think. Other genes suggest LUCA fed on hydrogen, which is in line with previous studies. It may have been part of an ecosystem of other kinds of primitive cells that died out, the team speculates.
“I think it’s naive in the extreme to think that LUCA would have existed on its own,” says Donoghue.
The results also suggest it had a primitive version of the bacterial defence system known as CRISPR, to fight off viruses. “Even 4.2 billion years ago, our earliest ancestors are fighting off viruses,” says team member Edmund Moody, also at the University of Bristol.
“It’s almost certainly all wrong.”
To their credit, the researchers are demonstrating the kind of epistemic humility that is all-too-often missing in much science.
Peering back into the deep past is fraught with uncertainty, and Donoghue is the first to admit that his team may have missed the mark. “It’s almost certainly all wrong,” he says. “What we’re trying to do is push the envelope, and create the first kind of attempt at integrating all of the relevant evidence.”
“It won’t be the last word,” he says. “It won’t even be our last word on this topic, but we think it’s a good start.”
If only more scientists were as willing to admit that they might well be wrong.
Looking at you, ‘climate science’.
