LIFE × GENETICS
LUCA: the Last Universal Ancestor
~4.2 billion years ago (estimates 4.3 to 3.5 Ga)
LUCA, the last universal common ancestor, is the population of cells from which all life on Earth descends, and researchers reconstruct its characteristics by comparing millions of genes across the living world.
The last universal common ancestor, abbreviated LUCA, is the ancestral population of cells from which every living thing on Earth descends. No fossil of LUCA is known, and researchers describe it by a method called phylogenetic reconstruction. They compare millions of genes across the living world and retain what these genes have in common. The principle is that a feature shared by all descendants was most probably present in the ancestor. The cell reconstructed in this way lived on hydrogen and carbon dioxide, lived without oxygen, and already had genes for some 2,600 proteins. LUCA lived alongside other organisms, and its waste products could support a modest ecosystem around it.
Why it matters
LUCA is the earliest point that the reading of genomes can reach, and it comes after the origin of life. A genome is the complete genetic material of an organism. For the time before LUCA, the evidence is chemical. From LUCA onward, the evidence is genetic, and every branch of the tree of life leads back to LUCA.
Dating & uncertaintyDisputed
The age of LUCA comes from a computation, since no fossil of LUCA exists. One method uses genes that had already been duplicated before LUCA, so that each of the two copies helps to date the other. The method is calibrated, that is, tied to known ages, with later microbial fossils and with isotope records, the measurements of isotopes, forms of an element of different mass, in ancient rocks. It gives an age of 4.09 to 4.33 billion years. Earlier analyses rested on the fossil record and on the assumption that life could not have survived the early bombardments, the impacts of asteroids on the young Earth. They placed LUCA several hundred million years later. No rock settles the question. The gap matters, because at the older date life already existed when the first oceans had barely formed.
Earth history as one day
01:47:5022 h 12 min before midnight
if Earth’s 4.54 billion years were compressed into a single day