Expert: John Shea - 1/27/2010

Question
Was the first modern human given birth by a more primitive human? If so, where did the second modern human come from and how did they find each other to make more modern humans? Was there some genetic fluke happening simultaneously in all the more primitive humans giving birth to the modern humans? I guess what I am trying to figure out how we went from just one modern human to 7 billion of us.
Thanks

Answer
Jeff
This is why, about 10 years ago, I made a pledge to stop using the term, "modern humans".
The problem with this question is that you are treating modern and primitive humans as real categories.
They aren't. These are crude and poorly-defined categorical distinctions that exist only in the imaginations of anthropologists.
Among all humans and human ancestors there is genetic, morphological, and behavioral variation.  Most of these differences are continuua -that is they are
continuously-variable (like a light spectrum) not discrete, categorical differences.
Parents of that first "modern" human -Let's call her "Betty"- differed from each other and differed from Betty.  Betty was more like them than she wasl
like her great-great-great grandparents in much the same way you and I differ our great-great-great grandparents.
Where people get confused about this is that they equate ancestry with existence.  700,000 years ago, one of more hominins were alive who have descendant
living today, but not all of those hominins had descendants (in much the same way my brothers have kids and I do not).  Molecular
anthropology can trace ancestry, but it only detects the legacy of ancestral individuals.   Whenever those ancestral individuals lived, there were always
more of them than have descendants among us.
The numbers of ancestral humans who were alive at any given time in prehistory varied, but for much of the Pleistocene, there were probably at least a million
or so. What varies is the proportion of that million who have descendants.  The further back you go in time, the smaller the proportion of the ancestral population
(the proportion, mind you, not the population from which that proportion is drawn).
The rise from less than a million to 7 billion is a whole different matter -a question beyond my time available to answer. My opinion -it has to do with our
ancestors creating uniquely broad and stable ecological niche.

Cheers,
John Shea