Sunday, July 04, 2010
Early Humans in Ethiopia and Cagayan
The July 2010 cover of the National Geographic has the skull of a female Ardipithecus ramidus which is the oldest known hominid skeleton. For those who aren't familiar about the taxonomy and systematics of humans, Hominids are the great apes who belong to the family Hominidae. These include humans, chimpanzees, bonobos, gorillas, and orang utans. The family is further subdivided into Ponginae which contains the Orang utan and the Homininae which contains the chimps, bonobos, gorillas and humans. Gibbons belong to another ape family, the Hylobatidae.
Ardipithecus ramidus "Ardi" is the oldest known human ancestor with a fairly complete skeleton that has survived to the present. Estimated at 4.4 MY, the skeleton was unearthed in the Middle Awash in Ethiopia. Before the oldest fossil skeleton was the famous Lucy (Australopithecus afarensis) discovered by Donald Johansen in the Awash region of Ethiopia in 1974 and is estimated to have lived 3.2 MYA. The skeleton was nicknamed "Lucy" because the Beatles hit "Lucy in the Sky with Diamonds" was often played at the dig.
While Lucy was fully bipedal like us, Ardi was not. Ardi was quadrupedal in the trees and clumsily bipedal on the ground. While Lucy's skeleton looks more that of a human (note the pelvic bones), Ardi looks like a suite of primitive chimp like skeletal features mixed in with more modern features. Looking at a photo of Ardi's skeleton, I can't characterize it as a chimp or a gorilla but something else. Ardi's hand is unusually long for her height. The hand is the same size as mine, but Ardi was just a wee bit shorter than 4 feet in height and her long arms and bodily proportion is not human more like that of a chimp's. Her foot is definitely unlike ours, it looks more like of a chimp's than ours. The opposable toe allowed her to have gripped tree branches as she moved in the trees. But she is no chimp. She unlikely did knuckle-walking on the ground. Also like in all human species, Ardipithecus did not have large canines especially in males.
Ardi lived at a time when East Africa was a wetter and more forested place. Climate change a million years later resulted in drier open woodland savanna where Lucy and her kind lived. Ardi's diet is more vegetarian as compared to later hominids which were more carnivorous.
The discovery of Ardi in 1993 and its formal description in 2009 is a major scientific discovery and validates many earlier hypotheses in human evolution. It supports the idea of climate change as a driver for human evolution. It also is an excellent example of a species showing morphological changes from the more ape like to the more human like. But Ardi's brain was still the size of that of a chimp's.
Human evolution is a good example of species diversification as natural selection went into high gear. But now only one species of human survives, whereas there were more than 20. As late as 15K years ago, we were not the only human species on Earth. There was the "hobbit" Homo floresiensis in Indonesia.
This is where University of the Philippines professor of archaeology Dr Armand Mijares' latest discovery of a human toe bone in Callao Cave, Cagayan province becomes significant. His research team just published in the Journal of Human Evolution their age estimate of the fossil which is at 67K years old. This pushes back the history of humans in the Philippines. Previously the oldest known human fossil was Tabon Man's skullcap estimated at 22K-30K years old and belongs to our species.
That Northern Luzon is the site where the fossil was discovered is extremely interesting for me as a biogeographer. It means that humans were able to cross over from Mainland Asia to Luzon. Luzon was never connected to Asia. Early humans would have taken a boat to get there. They could have walked over to Palawan (which was connected to Borneo for a time), took a boat to Mindoro, crossed over and successfully navigated the strong currents of the Verde Island Passage (ask any SCUBA diver what this means), and landed in Batangas, walked to where Manila is now onto central Luzon, cross the Caraballo Mountains and get to the Cagayan Valley.
Why would they do this? Perhaps they were following large game. We know that elephants and rhinos once roamed Luzon.
Mijares and his team reports that the toe bone is of the gracile type and suggests that it came from a human with a small frame. Intriguingly it falls within the measures of the foot bones of Homo floresiensis or Homo habilis. But habilis is an African species that lived 2.3 to 1.4 MYA. So it is unlikely that it belongs to that species. It can be a dwarf human species like floresiensis or a small Homo sapiens. These questions can only be answered if Mijares and his team finds more bones, most especially a skull with jaws.
If it is really a species of dwarf human, it would be of immense scientific significance. If we relate this to the biogeography of the Philippines, we know that each island of the archipelago has its own endemic species of mammals. Some of the mammals have undergone dwarfism like the Tamaraw of Mindoro or the fossil Cebu dwarf buffalo (even smaller than the Tamaraw!). If humans reached the larger islands like Cebu, Negros, Panay, Mindanao and they were isolated, they could have evolved in isolation and became small sized.
Each Philippine island may have had its own species of human! This is not far fetched. More intensive archaeological and paleontological research in Southeast Asia may result in the discovery of more human species. But the problem is that the wet and humid tropical climate is not suitable for fossilization unlike in the dry deserts of Ethiopia. Thus Mijares' discovery is even more remarkable.