Richards (2006) Homo floresiensis reconsidered

[Daryl Habel]

A new paper offers a different hypothesis, a twist we haven't heard previously, on the nature of the Liang Bua hominins: " that the remains represent a variant of H. sapiens possessing a combined growth hormone–insulin-like growth factor I axis modification and mutation of the MCPH gene family" (Richards 2006).

Richards GD. 2006. Genetic, physiologic and ecogeographic factors contributing to variation in Homo sapiens: Homo floresiensis reconsidered. J Evol Biol (online before press).

A new species, Homo floresiensis, was recently named for Pleistocene hominid remains on Flores, Indonesia. Significant controversy has arisen regarding this species. To address controversial issues and refocus investigations, I examine the affinities of these remains with Homo sapiens. Clarification of problematic issues is sought through an integration of genetic and physiological data on brain ontogeny and evolution. Clarification of the taxonomic value of various 'primitive' traits is possible given these data. Based on this evidence and using a H. sapiens morphological template, models are developed to account for the combination of features displayed in the Flores fossils. Given this overview, I find substantial support for the hypothesis that the remains represent a variant of H. sapiens possessing a combined growth hormone–insulin-like growth factor I axis modification and mutation of the MCPH gene family. Further work will be required to determine the extent to which this variant characterized the population.

The abstract and subcription access from Blackwell Synergy is available HERE, but  if you know where to look, it's floating around in the files of various discussion groups.

Comment by John Hawks on his weblog: CLICK HERE

and comment by Carl Zimmer  (including a response from Peter Brown) on "The Loom": CLICK HERE

And the beat goes on,
Dar

[Daryl Habel]

As I read this new paper:

I think Richards is suggesting microcephaly (by definition, s.l.),
but not the same kind of microcephaly as has been previously suggested
(secondary microcephaly).  Here's what Richards says: "Because the
morphology of the LB1 skull is unlikely to be the product of one of
the secondary causes of microcephaly (contra Hernneberg and Thorne
2004), it is necessary to focus on primary causes" (Richards 2006:
page 7).

Richards suggests the "primary causes" of microcephaly fall into two
subcategories: 1) MCPH (autosomal recessive primary microcephaly
MCPH1 to 6), with "relatively normal gyral pattern", and 2)
MCsg, "microcephaly with simplified gyral pattern".

Richards concludes LB1 falls into the MCPH family and discusses this
under the subtitle "Growth hormone-insulin-like growth factor I
axis" (page 9), quoting:

"It is clear that significantly reduced brain sizes can be produced
by a group of reasonably common genetic changes that affect neuronal
precursor cells and result in reduced cerebral cortex size.
Furthermore, a direct correlation between the degree of brain size
reduction and the degree to which mental capacity is impaied in
these cases does not appear to exist. These data confirm that human
brain sizes in the range suggested for H. floresiensis are not
necessarily incompatible with the acquisition of basic survival
skills..."

So here we have a mechanism, but I'm sure detractors will come up
with... "yes, but what are the odds that LB1 is such and that this
rarity would be the example excavated?" To address this issue,
Richards closes the above paragraph with:

"...They also indicate that such malformations are highly likely to
arise in small, isolated populations such as those inhabiting remote
islands" (Richards 2006: p. 9).

So Richards is maintaining microcephaly (which, by definition,
it is), but suggests a possible cause which does not require a
separate species, adding that the individual, LB1, afflicted with this
microcephaly could acquire the survival skills to reach maturity.

At least that's how it reads to me.  But I don't expect this to end
the controversies surrounding H. floresiensis.
Dar