Who are we all descended from? And I mean 𝑎𝑙𝑙 - people, birds, plants, bacteria, archaea....

It's called 𝗟𝗨𝗖𝗔: 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 CO₂ + 4 H₂ → CH₃COOH + 2 H₂O

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 (CH₄) into the atmosphere, which gets broken down to H₂ by sunlight. When this dissolved in water, acetogens can eat it!

The paper is open access:

• 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