When DID "life" begin?

[muon2]

I think the modern answer to the question in the OP comes by reframing it this way, "When did the last common ancestor of all living humans live?" It defines what biologists call the clade of humans. The dominant view among biologists would place the answer at around 200,000 years ago.

[muon2]

Species are pretty hard to define. There are scientific debates over how broad or narrow one should define it, and when one species transitions into another.

That's why modern biology prefers to speak of clades - a group of all organisms that share a specific common ancestor. It gets away from the older definitions of species and removes the less important biological question of where one species ended and another begins. It replaces the definition with the more important question of how much genetic material is shared between two organisms and what other organisms have similar baseline genetic material. Often a species is a clade, but there are clades made up of multiple species and clades within a species.

[muon2]

I think the modern answer to the question in the OP comes by reframing it this way, "When did the last common ancestor of all living humans live?" It defines what biologists call the clade of humans. The dominant view among biologists would place the answer at around 200,000 years ago.

I don't this makes sense on the micro scale, with sexual reproduction involved. The last common ancestor of all living humans lived very recently, considering that they weren't the only ancestor of all iliving humans. Wikipedia says that the last common ancestor of all living humans might have beeen as recently as 3000 years ago, but it would be ridiculous to say that that person was the first human.

I understood that the MRCA paper that set it at 3000 years ago was one primarily based on statistical modeling assuming a certain rate of interbreeding. The older dates refers to the genetic and female MRCA. In 2013 those dates were generally between 100K and 200K years ago.

I understand that when lines die out the MRCA date moves forward in time. One could include all modern humans living and dead in the clade and that keeps the MRCA date fixed further back.

There's a lot of research currently debated about the potential interbreeding between humans and other homo populations like the Neanderthals and how that might affect the MRCA date. A clade with all of those populations can be defined, too.

[muon2]

I think the modern answer to the question in the OP comes by reframing it this way, "When did the last common ancestor of all living humans live?" It defines what biologists call the clade of humans. The dominant view among biologists would place the answer at around 200,000 years ago.

I don't this makes sense on the micro scale, with sexual reproduction involved. The last common ancestor of all living humans lived very recently, considering that they weren't the only ancestor of all iliving humans. Wikipedia says that the last common ancestor of all living humans might have beeen as recently as 3000 years ago, but it would be ridiculous to say that that person was the first human.

I understood that the MRCA paper that set it at 3000 years ago was one primarily based on statistical modeling assuming a certain rate of interbreeding. The older dates refers to the genetic and female MRCA. In 2013 those dates were generally between 100K and 200K years ago.

I understand that when lines die out the MRCA date moves forward in time. One could include all modern humans living and dead in the clade and that keeps the MRCA date fixed further back.

There's a lot of research currently debated about the potential interbreeding between humans and other homo populations like the Neanderthals and how that might affect the MRCA date. A clade with all of those populations can be defined, too.

Sure, but I don't think using the MRCA to define "human" is a good idea, considering that there were individuals alive before the MRCA who were clearly human under any reasonable definition, and there was nothing unusual about the MRCA other than being, uh, very sexually active.

I agree, which is why I would use the MRCA of all identified anatomically modern humans living and dead. A genome extracted from human remains from 45K years ago is a legitimate source for comparison to today's populations.