Monthly Archives: November 2009
Every November, evidently, I will wake up and remember I have a blog. And then when November’s nearly over, I will consider updating it.
In keeping with the journalism theme, let my first post of 2009 be this:
Discovery of early hominid suggests new possibilities for human evolution
We have long known that the modern human didn’t evolve from primates, but rather shares a common ancestor with them.
Many scientists have long assumed that our closest relatives currently inhabiting planet earth, chimpanzees and gorillas, represent a sort of window into the past. However, new evidence has come to light suggesting that primates today are not relics of primitive human-like species, but rather the descendents of a related, but separate, evolutionary line from humans, potentially as different from the common ancestor as humans are.
The first thorough description of Ardipithecus ramidus, a hominid species that lived 4.4 million years ago in what is now Ethiopia, was published on October 2 in the journal Science, run by the AAAS, a nonprofit science society. It provides fossil descriptions and botanical analysis of the archaeological site, the results of which challenge many current assumptions about the history of human evolution.
The results of these studies have been 15 years in the making, as an international team of scientists carefully examined findings from 1992 and 1994 excavations, which consisted of a collection of bones comprising about 110 individuals, including a nearly complete skull with most of its teeth.
The most complete skeleton is believed to be a female based on teeth and brow ridge characteristics, but overlap in body size between male and female A. ramidus makes this less certain. Nevertheless, “she” has come to be known as “Ardi,” and she represents the discovery of a previously unknown phase of evolution.
Ardi weighed 50 kilograms and was about 120 cm tall (that’s less than four feet). That’s almost two times heavier, and one foot taller, than the famous Lucy, who, before Ardi, was the earliest known human ancestor.
Lucy, discovered by American anthropologist Donald Johanson in 1974, is a 3.2 million-year-old Australopithecus afarensis skeleton. Lucy, like Ardi, was a small-brained primate who, unlike Ardi, exhibited a more advanced form of upright walking, and spent very little, if any, time on all fours.
Ardi, like modern primates, has a grasping big toe or, in other words, a foot that looks much more like a hand.
However, unlike modern primates, she shows no evidence of knuckle-walking, and was capable of both tree climbing and upright walking, even though evidence shows the trees were still in the environment.
This suggests that the move away from four legs and tree life was more an issue of increased sociability, than of adaptation to a new, treeless environment.
That Ardi shows both primate and hominid traits, while also being a precursor to Lucy, is an important point in the way we view evolution.
Rather than a straight line with successive traits being changed bit by bit, Ardi demonstrates the flexibility of evolution.
One example is the existence of parallel evolution, which is when species evolve similar characteristics to each other independently of a common ancestor. This is also known as convergent adaptations.
Ardi is more similar to humans’ and primates’ last common ancestor than she is to modern chimpanzees, which show that chimps have evolved too.
Some limitations are assumptions about the regularity of molecular change, and the reliability of current dating methods, which both have strong limitations
The last common ancestor is thought to have lived six to seven million years ago. Our most recent human ancestors, of the genus Homo, lived about two million years ago.
Some theories place Ardi as a direct ancestor of Lucy, others suggest she may be from a different, although related line. However, the basic order is unknowable without a more complete fossil record. There are still huge gaps in our ancestral timeline, gaps between locations where bones have been found, which means many assumptions have to be made, and few theories can as of yet be viewed with any certainty.
Although science can sometimes seem to move by leaps and bounds, and indeed, as Ardi fills in one more link in the evolutionary chain, it is clear that there is still so much more to be discovered.