More Hobbit press, and “reverse evolution”

There’s a nice article in the New York Times about the Hobbit, written by John Noble Wilford, who seems to cover the human evolution beat pretty well.  It’s a nice summary of what the major hypotheses are and have been over the past few years.  Particularly, they address the idea that H. floresiensis might be the last vestiges of an ancient migration out of Africa by one of the Australopitecines or maybe Homo habilis.

There’s always that possibility.  I’m certainly not ready to discount that it may be a pathological human, or even a pathological H. erectus, but I also haven’t been keeping up with the Hobbit literature as much as I probably should be.  An ancient migration out of Africa is usually dismissed as impossible because the little Australopithecines didn’t have long enough legs to make long-distance migration efficient.  I’ve always found that argument a little confusing, though.  It’s not as if the individuals themselves had to have been walking long distances.  They could have been just expanding the outskirts of their range, little by little with each generation, or even season.  And one needs only to point to the New World Monkeys to show that long-distance migrations can indeed occur with small bodied animals, so long as sufficient resources are present on a natural raft.

The article is good, but one paragraph did jump out at me and send me into nit-pick mode:

Or could the hobbits be an example of reverse evolution? That would seem even more bizarre; there are no known cases in primate evolution of a wholesale reversion to some ancestor in its lineage.

I’m not sure why this is in here.  Nobody thinks that the Hobbit is “reverse evolution,” because such a thing doesn’t exist.  While some interpretations conclude that Flo has an australopithecine-like wrist, that doesn’t mean that it’s identical to an australopithecine, or even that the wrist is identical to an australopthecine’s wrist.  What it means is that, in a statistical analysis, the capitate groups more closely with australopithecines than with modern humans.  The Hobbit’s trapezoid also groups more closely with those from chimps and gorillas than it does with humans, but again, this doesn’t mean that it was identical.

Furthermore, not only are there no cases of “wholesale reversions” in primate evolution, but I’ve never heard of such a thing happening in any lineage at all!  Sure, there are atavisms, or reversions back to a “primitive” trait, but those result in a descendent with an ancestor-like trait, not a perfect duplicate of the ancestor.  “Reverse evolution” doesn’t happen.  Once an animal evolves, it can’t un-evolve.  It can loose traits, or re-evolve a basal trait, but a human isn’t going to “reverse evolve” into an australopithecine, and a bird isn’t going to “reverse evolve” into a dinosaur.  That bird may re-evolve teeth, but that wouldn’t make it a dinosaur.  The analysis of the wrist may lead us to the conclusion that the ancestor of the Hobbits left Africa before our modern wrists had evolved.  It may even lead us to the conclusion that the Hobbits left Africa with a modern wrist and then evovled a wrist that looks more primitive, but even that’s a stretch.  What it doesn’t lead us to conclude is that the ancestor of the Hobbits left Africa and then reverse evolved back into Australopithecines.

References:

Tocheri et al. (2007) The Primitive Wrist of Homo floresiensis and its Implications for Hominin Evolution.  Science 317: 1743-1745.   DOI: 10.1126/science.1147143

News from Zhoukoudian

Peking Man Exhibition Site, Ca. 1930

Zhoukoudian is famous for being the cave system near Beijing in which some of the very first human fossils were found.  These fossil are sometimes referred to as those belonging to “Peking Man,” but most of us now just call them Homo erectus. The original fossils found at this locality were lost at sea during World War II, but since then, many new fossils have been discovered here.  Previous estimates have dated the hominin-bearing layers of the Zhoukoudian cave sites to between 230,000-500,000 years old, but new research indicates that these sites may be as old as 770,000 years.

The new technique used here is called cosmogenic 26Al/10Be burial dating, and is an aluminum/beryllium radiometric technique.  Aluminum and Beryllium exist in quartz grains found in the sandy bottoms of the caves, and each decay at known rates from exposure to cosmic radiation (The half-life of 26-Al is 730,000 years, and that of 10Be is 1.51 million years).  Scientists can compare the ratio of these isotopes to find out when the quartz sediments were buried under other sediment- an event which stops the isotopes from decaying since it halts the exposure to cosmic radiation.  Previous estimates have been based on magnetostratigraphy, in which the known dates of reversals of the Earth’s magnetic field are used to date sedimentary rocks and the associated artifacts.

Fossil evidence indicates that the first Homo erectus lived in Africa around 2 million years ago.  They had made it to Dmanisi, Georgia by about 1.75 million years ago, and Java by 1.6 million years ago.  Homo erectus was certainly a species capable of surviving in many different environments as it made its way across the world.  At Zhoukoudian, other mammalian fossils indicate that while Homo erectus was living there, there were periods of warm weather alternating with periods of cold weather, with an overall trend toward cool, dry grasslands. These grasslands were very rich in mammal species which Homo erectus could hunt (or at least take advantage of after other species had “hunted” them), making Ice-Age China a very comfortable place.

Shen G, X Gao, B Gao, and D Granger.  Age of Zhoukoudian Homo erectus determined with ²⁶Al/¹⁰Be burial dating. Nature.  458: 198-201.

Image Credit:  Bernard Becker Medical Library, Washington University in St. Louis.

The New Homo erectus pelvis from Gona

A group of scientists working in the Afar region of Ethiopia have found a nearly complete female Homo erectus pelvis from around 1.8 million years ago.

The pelvis includes a sacrum, both os coxa, and a complete pubis.  This pelvis is much more complete than the Turkana boy pelvis (KNM-WT 15000), which has been the main model for H. erectus pelvis and body shape.  The Turkana pelvis has been reconstructed as tall and narrow, which is said to be an adaptation to long-distance running as it would’ve increased the efficiency.  Turkana boy has been reconstructed to stand about 5′3″ at an age of about 11.  Had he lived into adulthood, he would’ve been around 6′1″.  Estimates of female pelvic shape derived from the Turkana boy have suggested that females would’ve had to have given birth to neonates with small brains, which would’ve required a long, human-like period of infant dependency.

The new pelvis from Gona permits a height estimation of between 3′11″ to 4′9″ based on the small size of the acetabulum (where the head of the femur articulates with the pelvis).  A small acetabulum reflects a small femoral head, which reflects a small body size.  If the Turkana estimates are correct, and the body size regression from the acetabular size is correct, sexual dimorphism in body height was probably greater in Homo erectus than has been previously estimated.  Still, these estimates for body size seem reasonable when I think of dimorphism in modern humans.

The pelvic inlet in the new pelvis is within the size range of modern human females, despite the fact that the individual was very short.  The outlet was even greater than that in modern humans.  These dimensions can be used to estimate the brain size of a neonate.  The new pelvis estimates brain size of 315 ml, which is around 30% greater than the estimates derived from the Turkana boy.  These estimates indicate that neonatal brain size in Homo erectus was between 34-36% the size of adults.  In chimpanzees, neonatal brain sizes are 40% that of adults, and in humans they are 28%.

An increase in brain size like that seen in Homo erectus requires an increase in size of the pelvis.  This can be achieved by either increasing the size of the body overall, or increasing the size of the pelvic canal only, and especially in the female of the species.  The new pelvis suggests that brain size was acting more on pelvic shape than on overall body size.

BSN49/P27 vs. AL288 (Homo erectus vs. Australopithecus afarensis) (From Simpson et. al 2008)

Modern human female pelvis

Since the Australopithecines, there have been many changes in pelvic shape.  The pelvis has become more circular, with a decrease in bi-auricular breadth and an increase in anterior-posterior breadth.  The pubic ramus has become shorter, which contributes to the decrease in biauricular breadth.  The ilia have become less flared as well, contributing to an overall narrower body shape.

Some authors have suggested that Homo erectus displayed these adaptations as adaptations to long distance running.  Characters such as tall body size, large acetabula (where the femur meets the pelvis), and narrow torsos and pelves are used as evidence that Homo erectus was increasing locomotor efficiency. However, the pelvis from Gona displays none of these adaptations.  Her bi-iliac breadth is wide, her ilia are flared laterally, her acetabula are tiny, and her pubic rami are long.

Clearly, something is amiss.  The body size dimorphism seems reasonable, but the pelvic shape suggests extreme behavioral dimorphism as well (males were out running long distances while the females waddled around bearing children at home). This may be reasonable, but it may not be.  Other Homo erectus specimens, like the ones from Dmanisi provide evidence that small body size in erectus was not unusual. Perhaps our reconstruction of the Turkana pelvis requires modification, as it seems to be an outlier.

Sources:

Simpson SW, Quade J, Levin NE, Butler R, Dupont-Nivet G, Everett M, Semaw S. 2008. A female Homo erectus pelvis from Gona, Ethiopia. Science 322:1089-1092.