John Carlos BaezAug 3, 2024

Who are we all descended from? And I mean π‘Žπ‘™π‘™ - people, birds, plants, bacteria, archaea....

It's called π—Ÿπ—¨π—–π—”: the last universal common ancestor. And people have been trying to track it down. By comparing the genomes of different organisms you can infer a tree of life and guess where it leads back to.

This new paper suggests that LUCA lived about 4.2 billion years ago, with a genome having about 2.5 million base pairs.

They guess it was a prokaryote: a single-celled organism with no nucleus. They guess it was anaerobic. Neither of those are at all surprising. More interestingly, they guess it was an acetogen! I hope you know acetic acid is what makes vinegar sour. Nowadays, 'acetogens' are bacteria that power themselves by converting carbon dioxide and hydrogen to acetic acid and water:

2 COβ‚‚ + 4 Hβ‚‚ β†’ CH₃COOH + 2 Hβ‚‚O

This produces less energy than fermentation, which converts glucose to acetic acid. But hey: sometimes there ain't no glucose around.

The paper says that the metabolism of LUCA could have provided a niche for other microbes living at the time, and recycled hydrogen they made. It's the blue box in the web of chemical reactions carried out by early organisms in the picture at left.

At right you see how if bacteria called methanogens were also also around, they could put methane (CHβ‚„) into the atmosphere, which gets broken down to Hβ‚‚ by sunlight. When this dissolved in water, acetogens can eat it!

The paper is open access:

β€’ The nature of the last universal common ancestor and its impact on the early Earth system, https://www.nature.com/articles/s41559-024-02461-1

The nature of the last universal common ancestor and its impact on the early Earth system - Nature Ecology & Evolution

Integration of phylogenetics, comparative genomics and palaeobiological approaches suggests that the last universal common ancestor lived about 4.2 billion years ago and was a complex prokaryote-grade anaerobic acetogen that was part of an ecosystem.

Nature
Show threadRowena Ball

@johncarlosbaez

Amazing, but not surprising, that the LUCA organism was already so sophisticated 4.2 billion years ago! I published a diagram similar to the authors' Fig. 3b in 2019. (It would be nice to think that it inspired the authors.)

The 'tree of life' is widely used both as metaphor and cartoon for the homology of life. Its branches vanish as they grow, so we can see and study directly only its existing tips and such vanished tips for which we have found fossils; the antecedent structure mostly is deduced.

That all life descended from a common ancestor has been widely accepted as a fundamental dogma of biology since Darwin and Pasteur, but this has been questioned. There would seem to be no good reason why cellular life should have arisen just once on Earth. Surely, a 'shadow biosphere' could coexist with the known biosphere - after all, the three domains of life rub along in more-or-less peaceful, often interdependent, coexistence to this day.

I used the diagram and evidence of the early evolution of catalases, from simple Fe^2+ through heme complexes to enzymes, to show that a shadow biosphere could not exist.

If the tree paradigm applies to evolution of the biosphere from LUCA onward, by continuity it must also apply to pre- LUCA evolution. So I also made one for a pre-LUCA tree of life! That was fun!

Aug 3, 2024 at 10:13amWeb