Question

How much do we know about ancient viruses and the viral evolution throughout the human history?

To my knowledge the HIV history has been rather well documented for about hundred years back (e.g., see here and here) - via the phylogenetic reconstruction, massive study of this virus since it was discovered, and the analysis of the records and blood samples from the countries where the virus first spread (before arriving to the West and being discovered).

Ebola has been well documented as well over the last half a century or so, although it hasn't changed much, since the repeated outbreaks are spills of very similar strains from the nature.

Finally, there have been obviously many viruses well documented since the advent of massive sequencing.

The question is thus about the viruses with longest history:

What are such well-documented viruses?
How much do we know about them?
How do we know this?
How much have they changed ? ( In terms of their danger to humans, e.g.)
Disclosure: the question is motivated by this one. Here is a somewhat similar question (but rather limited in scope).

Answer 1

Although viruses do not fossilize, researchers study ancient viruses and their evolution through biochemical and genetic information. Viruses such as HIV and Ebola are well-documented, but our knowledge of ancient viral history relies on indirect evidence like viral DNA remnants in host genomes. Multiple origin hypotheses exist, yet much about virus origins and evolution remains speculative.

Step-by-step explanation:

Exploring the evolutionary history of ancient viruses poses a unique challenge as viruses do not leave fossil records.

Instead, researchers rely on biochemical and genetic analyses to conjecture about their history.

We know more about recent viruses like HIV, which is well-documented and traced back to the early to mid-1900s when it likely made a zoonotic jump from primates to humans in West Africa.

Ebola, identified over the last half-century, hasn't changed substantially, with outbreaks stemming from similar strains recurring from nature.

For ancient viruses, the understanding is primarily speculative and indirect. Information about viral DNA, particularly from retroviruses, integrated into host genomes helps construct evolutionary histories, despite being incomplete and fragmented.

This approach allows scientists to examine evolutionary relationships and changes over time. Viral DNA sequences that have become part of host genomes can indicate how much viruses have changed and adapted.

There are several hypotheses regarding the origin of viruses, including the devolution or regressive hypothesis, the escapist or progressive hypothesis, and the virus-first hypothesis, yet there are no conclusive answers.

The study of viruses' evolution is ongoing and continues to reveal insights about their adaptability and potential threats to human health.

For example, ancient viral DNA found in human genomes provides evidence that human populations have managed ancient viral infections and co-evolved with their viral parasites.