What common ancestor does all life share?

Answer 1

The last universal common ancestor (or LUCA) for all known life would have been some single celled organism similar to prokaryotes. It might have been an RNA or DNA based organism, but it would have used more or less the same genetic code as all modern organisms (with a few minor variations) today. It will probably have lacked a true cellular nucleus, and many of the organelles that modern organisms have, but it would still have used ATP as a key factor in its metabolism.

Note that, like with all Most Recent Common Ancestors for any group, it is not necessary that a single species of organism is the sole ancestor for all modern life. It may well be possible that different species contributed to modern life (see also: multiple origins hypotheses). There will be one single ancestor common to any specific set of traits in all the diverging lineages, but, depending on the set of traits examined, they may lead to different origins. Compare, for instance, to mt-DNA Eve and Y-chromosome Adam: there is one most recent common ancestor for all surviving variations of mitochondrial DNA via matrilineal descent, but there is a different ancestor, living in a different era, for all variant Y-chromosomes surviving through patrilinial descent. It is, however, likely that there was, at some time, indeed one single species or closely linked collection of species of organisms from which all modern life descends. Also note that LUCA concerns only the most recent of such ancestral organisms: it itself would have had ancestors, and shared ancestors with other lineages existing at the time.