When did our genus arise?

Glenn R. Morton (grmorton@waymark.net)
Sun, 14 Jun 1998 16:22:14 -0500

I ran into the following in Colin Tudge's book which concerns what kind of
gap you can expect between the actual speciation event and the first
appearance of that species in the fossil record.

"Logic dictates, too, that the oldest known fossils cannot possibly be the
oldest representatives of their kind. Fossilization is a rare event, after
all; and when animals first appear, they are rare. The earliest fossil
bones are therefore likely to date from a time when their erstwhile owners
were already common. Logic similarly dictates that if an animal is
particularly unlikely to form fossils--as primates seem to be--then
paleontologists are particularly unlikely to find the very earliest types.
In fact, this logic can be translated into a mathematical formula (see
Robert D. Martin, "Primate Origins: Plugging the Gaps," Nature, May 20,
1993, pp 223-234). The fewer fossils there are (relative to the calculated
number of extinct species), the older the group is liable to be, relative
to the number of fossils found." ~ Colin Tudge, The Time Before History,
(New York:Scribner, 1996), p. 172

The article that Tudge cites, presents a statistical study in which a
model phylogentic tree is designed and 3% of the species in the full model
end up as fossils (this is consistent with fossil data) The phylogenetic
model begins at 16 million years ago. But due to the incomplete sampling
(i.e. fossilization) the earliest example of the model appears around 11
myr. This of course underestimates the time of origin for the phylogeny by
5 million years. This is a 30% underestimate in time for the origin of the

So when we find H. erectus being widespread at a little under 2 million
years, we can be very confident that he existed earlier. How much earlier?
We don't know but the earliest member of the genus Homo is dated at 2.3
myr (AL-666) Even a 30% underestimate moves the origin of our genus 3.3
Myr. And we must remember that there is a possibility that the
underestimate is bigger as is obvious in some of the cases I gave earlier

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