Genetic modelling adds a new twist

Just when we thought we were getting a firm grip on the place of the diminutive early human Homo floresiensis, better known as the hobbit, in our evolutionary family tree, new research keeps alive at least part of a long-held theory about the one-metre-tall hominid being a dwarf descendant of Homo erectus, the first hominid believed to have left Africa.

This hitherto widely accepted theory has been challenged of late. A study published in April – and credited as the most comprehensive analysis to date of the bones of H floresiensis – confidently reported that the tiny human – also known as Flores Man, because his remains were discovered on the Indonesian island of Flores in 2003 – was not descended from H erectus but most likely from another ancestor in Africa.

That study, led by Debbie Argue of the Australian National University (ANU), used 133 data points ranging across the skull, jaws, teeth, arms, legs and shoulders of the H. floresiensis fossil to conclude that many features were more primitive than H. erectus, and that therefore the hobbits were most likely a sister species of Homo habilis, one of the earliest known species of human found in Africa 1.75 million years ago. It was possible, Argue’s team said, that H. floresiensis evolved in Africa and migrated, or that a common ancestor moved from Africa then evolved into H. floresiensis.

Now José Alexandre Felizola Diniz-Filho of the Federal University of Goiás, Brazil, and Pasquale Raia of University of Naples Federico II have revisited the controversy through their application of quantitative evolutionary genetics modelling, using simulations to evaluate the possible trajectories of body dwarfing between H. erectus (with an estimated body size value of 50 kg) and H. floresiensis (with a body size value of 27 kg).

The pair state their results, published in the journal Proceedings of the Royal Society B, “do not say any final word” on the matter, but their analysis does “consistently support a relatively large-bodied hominid as the ancestor to H. floresiensis”.

If H. floresiensis originated from an early small-bodied Homo such as such as H. habilis, they note, its small body and brain “would just reflect deeper ancestry followed by little evolutionary change”. However, that still leaves “deep implications for the so-called ‘Out of Africa I’ hypothesis, which portrays H. erectus or related forms as the first hominin to leave Africa”.

On the other hand, the theory that H. floresiensis evolved from H. erectus also required understanding how Flores Man, given the small size not just of its body but also its brain, could be the evolutionary result of insular dwarfism due to the ‘island rule’, where the limited environment and resources of islands see smaller mainland animals become larger, while larger animals become smaller.

The pair report that the hobbit’s small body and brain size are “perfectly consistent” with dwarfing driven by strong directional selection under the island rule. “Our results also show that the exceedingly small cranial volume of H. floresiensis might have required additional and independent selective forces acting on brain size alone, reinforcing the role of energetic constraints underlying the island rule.”

Thus their findings also “support previous conclusions that H. floresiensis may be most likely derived from an early Indonesian H. erectus, which is coherent with currently accepted biogeographical scenario for Homo expansion out of Africa”.