All living beings on the planet have a common genetic heritage . Humans, for example, share more than 99% of the genome with chimpanzees, but also half with bananas. This has allowed experts to deduce that life on Earth had the same origin – that is, it arose only once – and that there must be a single ancestor of all the organisms that inhabit it, from bacteria to blue whales. This universal ancestor is known as LUCA, an acronym for Last Universal Common Ancestor . The researchers think that it could have arisen in environments similar to the geysers and geothermal sources of Yellowstone Park, in the United States (photo).
As its name suggests, it is the last common ancestor to all life forms, but it does not represent the earliest stage of evolution. What’s more, what happened before him is something that we still don’t know very well . On the other hand, that all living things share the same genetic code tells us nothing about the nature of LUCA.
We could start from this assumption: what traits are common to all cellular life? To determine this, perhaps we should compare the representative genomes of the three natural domains: archaea, bacteria, and eukaryotes . Genes that are common to all of them would also have been present in LUCA.
But this is easier said than done. One problem is that genes move from one organism to another, in a process called horizontal transfer . For example, 100 million years ago the bacterium Escherichia coli acquired at least 10% of the genome through more than two hundred such events.
It can also happen that certain genes that were in LUCA have stopped being universal , that is, that they disappeared from some domain. Some scientists argue that the aforementioned transfer is so important that, in practice, it is impossible to build evolutionary trees – something like genealogies – while others think that such a phenomenon is insignificant.
