Chance_Bluebird_5788 t1_iwulvth wrote on November 18, 2022 at 1:56 PM

The most recent common ancestor (MRCA) of any two extant species today are all dead. The theory of common descent says there was a MRCA of any two living organisms today, so the question is just what can we say about that MRCA, and with what level of confidence.

If we find a fossil that looks like a good candidate for the the MRCA, then we can use various radioactive dating methods or existing knowledge about the layer or environment it was found in to help estimate it's age.

The DNA method you're referring to works roughly by taking an estimate of how quickly mutations happen, seeing how different the DNA between the extant species is, and dividing the amount of change by the estimated rate of change to estimate a divergence date.

However, at the end of the day you're right, these are the best estimates we can come up with, and there is certainly a margin of error that's bigger or smaller based on the amount of info we have to work with.

_Jaquen_Hgar_ t1_iwzjqm1 wrote on November 19, 2022 at 4:07 PM

For any given gene the mutation rate will vary depending on how tightly “conserved” (I.e. constrained by necessity) that type of gene is. For instance the genes controlling the creation of certain cytochrome molecules used in cellular respiration are so highly conserved that they haven’t changed since our last common ancestor with the pea plant. Whereas genes controlling something superficial like skin pigmentation or shape of the pinna may diverge much more freely.

Prae_ t1_ix1bv9h wrote on November 19, 2022 at 11:55 PM

That's only one of the many challenges faced by molecular clocks, although really not the biggest one. By construction you're looking at the neutral mutations, at as many loci as possible. Modern, Bayesian statistical methods can account for different clock speeds in different genes and different species, and of course sequencing is cheaper than ever.

CaptainHindsight92 OP t1_ix5gwla wrote on November 20, 2022 at 10:07 PM

So apparently mice share a more recent common ancestor to humans than pigs but the generation times between mice and pigs are so different, surely mice have evolved more simply because it may have had 1 billion generations where as a pig may have only had 100 million? How does it take that into account surely it would suggest one had a less recent ancestor but in reality it just evolved more?

Prae_ t1_ix5my08 wrote on November 20, 2022 at 10:50 PM

Indeed, generation time is one of the big things to take into account. That's why in the early days, the estimates that the genetic clocks gave was only good enough for closely related species.

But the method has come a long way. They have "relaxed clocks" now, which take into account generation time and stuff (it's all Bayesian statistics which I don't really understand). When they compare to known fossil records, it broadly agrees. But I think even now you'd expect it to be more accurate when comparing rodents together than the whole of arthropods for example.

CaptainHindsight92 OP t1_ix8k58s wrote on November 21, 2022 at 4:03 PM

Thank you for reply it was very helpful! You don't have any recommendations to read about this do you? Where would I find information on fossil information, generation time and gene evolution? Thanks again!

Prae_ t1_ix8tv3l wrote on November 21, 2022 at 5:09 PM

Most of what I said comes from scientific reviews, so not super layman friendly.

The tree of life web project is maybe something I can point to if you are interested in phylogeny.

Otherwise, all I can say is look at papers using BEAST2, which is, from what I understand, the main tool used for inferring a philogenetic tree from DNA sequences.