K Barrett wrote:
Another thing was - I'm vaguely recalling this - that a lot of DNA in the
human genome is from viruses that have attached to it in the dim past,
raising the question(s) how do they know and what then is human?

In many cases, old viral sequence has features that scream "virus"
(long terminal repeats, etc) or can be identified by homology to
current, active viruses. But sometimes, it's not obvious that there is
a bright line dividing "human" and "virus." Viruses can pick up human
sequences. The reverse can also happen: sometimes, the evolutionary
history of a human gene includes insertion of viral elements. For
instance, the salivary amylase gene appears to have been generated when
a retroviral sequence inserted into the promoter region of an ancestral
pancreatic amylase gene. The retroviral sequence is what causes the
amylase gene to be expressed in salivary glands. So, in this case, the
sequence of a virus has become human.