While I haven’t worked in the field for some years – as you know now, I make my living as a journalist – I was educated in Biology, and try to stay current, as I find the study of life fascinating. So I was interested, the other day, to stumble across some new research into the Last Universal Common Ancestor – LUCA – the organism from which all life present today on Earth is thought to have descended.
And, it turns out LUCA may have been a little more complex than we had thought.
LUCA does not represent the origin of life, the instance whereby some chemical alchemy snapped molecules into a form that allowed self-replication and all the mechanisms of evolution. Rather, it’s the moment when life as we know it took off. LUCA is the furthest point in evolutionary history that we can glimpse by working backward from what’s alive today. It’s the most recent ancestor shared by all modern life‚ our collective lineage traced back to a single ancient cellular population or organism.
“It’s not the first cell, it’s not the first microbe, it’s not the first anything, really,” said Greg Fournier (opens a new tab), an evolutionary biologist at the Massachusetts Institute of Technology. “In a way, it is the end of the story of the origin of life.”
Here’s where it gets interesting:
The analysis, published in Nature Ecology and Evolution in July, sketched a surprisingly complex picture (opens a new tab) of the cell. LUCA lived off hydrogen gas and carbon dioxide, boasted a genome as large as that of some modern bacteria, and already had a rudimentary immune system, according to the study. Its genomic complexity, the authors argue, suggests that LUCA was one of many lineages — the rest now extinct — living about 4.2 billion years ago, a turbulent time relatively early in Earth’s history and long thought too harsh for life to flourish.
What’s interesting – at least to me – is that LUCA wouldn’t have been the only life form around at that time, so long, long ago. There may well have been other organisms around; in fact, there almost certainly were. It’s kind of a fascinating notion. What would those other life forms have been like? What was their biochemistry? How did they differ from that single-celled proto-organism that gave rise to everything living on the planet today?
Some of them may have been viruses, which are alive – sort of.
LUCA may have dined on the chemical waste of other creatures, suggesting that it was already part of a complex ecosystem with other microbes. The analysis can’t provide direct evidence of this possible ancient ecology, since traces of such lineages are long gone. But it’s unlikely that LUCA would have evolved complexity in isolation, the authors argue. Since its metabolism is consistent with both relying on and potentially supporting other microbes, LUCA was likely part of a broader ecosystem, from which its lineage was the sole survivor.
At the very least, that ecosystem contained viruses: The researchers say LUCA likely housed 19 CRISPR genes, which bacteria use to slice up viral 
threats.
It’s a fascinating read, at least, for a biologist. But there’s a precautionary note that must be brought up: When you’re examining the possible structure and metabolism of something that lived – more or less – 4.2 billion years ago, there’s a lot of estimations and even some outright guesses.
But we do know that there was a LUCA. The question then becomes what form it took. And this, to me at least, looks like as reasonable a candidate as any.
Biologists will keep looking. I’ll keep reading.
