Friday, August 12, 2011

No Support for Expensive Tissue Hypothesis in Platyrrhines

Kari Allen and Rich Kay report in the Proceedings of the Royal Society B on a study of the relationship between dietary quality and relative brain size in platyrrhine primates (New World monkeys).  This study is relevant to human evolution because of a big idea in paleoanthropology known as the expensive tissue hypothesis (ETH, Aiello & Wheeler, 1995). The ETH predicts that dietary quality and relative brain size should be correlated.  As the picture at the top of this post shows, the idea is that low quality diets (e.g. leaves) are a constraint on primates, which have to devote lots of energy to make big guts capable of digesting these difficult foods. With so much energy invested in making a big gut, there isn't enough energy left over to make lots of another metabolically expensive tissue...brains.  The story as relates to humans is that humans had this constraint lifted at some point in their evolution, perhaps related to an increase in easy-to digest meat acquired with the help of tools.  Humans then devoted their excess energy -- no longer needed to make a big gut -- to make a big brain. 

However, Allen and Kay find no relationship between a measure of dietary quality and relative brain size in New World monkeys.  In particular Brachyteles has a very low dietary quality and a relatively large brain, going against predictions. This doesn't necessarily mean that the ETH isn't valid for humans, but it is certainly worth thinking about why the underlying relationship doesn't appear to hold for platyrrhines. 

Thursday, August 11, 2011

Participant Observation Meets Primatology: Why do chimps build sleeping nests in trees?

Fiona Stewart was interested in knowing why great apes sleep in nests at night.  There are a couple of hypotheses which have been floated regarding nest function: sleep quality, anti-predation, anti-parasite, and thermoregulation.  To test these hypotheses, Stewart went out in Fongoli, Senegal and actually slept in nests that chimps had used the night before, in self-constructed replicas, and on the ground.  She recorded data on sleep quality, the number of bug bites per hour of sleep, and recorded mean temperature at ground height and at nest height during the night. Results are published in a brief communication in AJPA

The results are very preliminary, but her sleep was less disturbed in nests, she suffered fewer insect bites, and the nests appeared to provide some insulation from the cold.  Perhaps the most telling result was in terms of sleep quality:
"All experimental nights’ sleep in this study were uncomfortable and characterized by low sleep quality (<0.50), compared with mean sleep quality recorded for captive chimpanzees (0.86) and human societies (Videan, 2006). "

Wednesday, August 10, 2011

Latest installment in the "hobbit" saga

LB1 (left) next to a modern human skull
Vannucci and colleagues just published the results of a study comparing a couple of brain measurements between microcephalic modern humans, normal modern humans, Homo erectusAustralopithecus and LB1 (the most complete specimen of the recently named species Homo floresiensis).   They argue that their results support the idea that LB1 is a microcephalic modern human, not a distinct species.  I am no brain expert, but I don't really think this study puts forward a slam-dunk argument.  For example, in the measurement that they focus on (cerebellar protrusion), LB1 doesn't seem to me to fit in very well with either microcephalic or normocephalic modern humans.  It is also interesting that they publish a cluster analysis based on their data which shows LB1 clustering more closely with Australopithecus than with microcephalic moderns.
Vannucci et al. 2011, PNAS.  Figures 6 (left) and 5A (right)
LB1 was shocking when first announced....a tiny hominin with an even tinier brain living on the Indonesian Island of Flores as recently as 18,000 years ago.  If this represents a species distinct from H. erectus and H. sapiens (as I would say a majority of the experts currently believe) then it is extremely interesting.  Prior to the discovery of LB1 in 2003, our understanding was that at 18,000 years ago, H. erectus should have been long since extinct, and the only living hominin species should have been modern humans.  The debate over LB1 has been between two camps: one arguing that the Flores hominins are a pathological population of modern humans (with microcephaly) and the other camp arguing that LB1 represents a non-pathological, distinct species of human ancestor that was previously unknown to science.

Bill Jungers clearly isn't buying it the conclusions of this most recent paper.  Jungers has been an outspoken proponent of the view that LB1 is non-pathological.  He makes his opinion crystal clear near the end the of this news piece in Nature.
"[Vanucci et al.] note a fascinating similarity in the cranial measurements found in Homo floresiensis and Australopithecus but ignored it in favour of making the microcephaly argument," he says. "A weird decision, but hobbit politics as usual. 

Tuesday, August 9, 2011

Hyaena turds hold many secrets

Some of the coprolites found at the site.
This post is about fossilized poop (called coprolites). Pesquero and colleagues report on an exceptionally rich assemblage of hyaena coprolites (nearly 1000 specimens) from the site of La Roma 2 in Spain. Based on their size and morphology the authors attribute the assemblage to the Late Miocene hyaenid species Lycyaena chaeretisCoprolites often preserve fossil pollen, which make them useful for paleoenvironmental reconstruction. 

A spotted hyaena eating a flamingo.

Sunday, August 7, 2011

The Savanna: Reloaded

Cerling et al report in Nature on the results of a survey of modern soil carbon isotope profiles from East Africa. They compare the modern results to a very large sample of fossil soil (paleosol) samples from the Awash valley and the Omo Turkana basins.  These two valleys in Ethiopia collectively sample approximately the last 6.5 million years of human evolution.  Their results indicate that open environments (not forests) have dominated these two valleys over much of the course of the last 6.5 million years.  This study is also part of the ongoing debate over the paleoenvironment of Aramis, the site from which Ardipithicus ramidus is best known.   Tim White and the Ardi crew argue that the Aramis site was heavily wooded and, further, that Ardi puts the coffin nail in the savanna hypothesis. Cerling et al. replied, arguing that the Aramis isotope evidence indicated more open environs than White and colleagues claimed.  This most recent paper provides evidence consistent with the interpretation that the Aramis site was pretty open, even more open than later A. afarensis sites.