Scientific America Dec 97 Ancestral Quandary by Kate Wong
In July of 1997, an article was published in the journal Cell which presented an analysis of DNA from uncovered remains of ancient humans. According to the report, which focused on evidence gathered from testing a bone from a Neanderthals arm, the DNA suggested to the research team that there is a definite gap between the Neanderthal and modern mankind. Furthermore, they stated that this proved that the two groups were not linked on the evolutionary chain. This find pushed ahead the theory that all modern humans have an origin from Africa. However, shortly after the article was published, several paleoanthropologists and geneticists came forward to rebuke that claim.
The theory, known by many as the out of Africa theory, was put forth by Londons Natural History Museums Christopher B. Stringer. It claims that modern mankind arose in Africa ca 130,000 to 200,000 years ago. From there it spread around to different continents. The hypothesis which challenges this idea is that of Milford H. Wolpoff from the University of Michigan. His states that 2 million years ago humans began developing within multiple populations connected by their genes and cultural exchanges, dispersing from there.
The recent conclusions based on the DNA from the arm bone were of the mitochondria or energy-producing organelles. When this DNA is passed from mother to child, it undergoes a mutation of sorts which occur at a constant rate which can be calculated. Researchers can then plot out a lineage of the genes.
The out of Africa, not-related-to-the-Neanderthal theorists concluded that there was a variation between Neanderthals and Moderns which was four times greater than any found in comparing to Moderns. They were also unable to match the mitochondrial gene to any modern Europeans. However, the gene segment which was being studied was very short and much less informative than a longer one might have been. In addition, the evidence was based on only one individuals genes. There might be a huge gap in that particular case, but there is no proof of that carrying over into entire populations.
Many scientists agree that the origin of our species is neither one nor the other but rather a combination of the two. The arguments will surely remain unsolved without a definite answer in the near future.
Wong, Kate. Ancestral Quandary, Scientific American.
A fossil recently discovered in Madagascar, which had feathers like a bird and claws like a dinosaur, has been dating to the Late Cretaceous period. The find adds to evidence linking birds and dinosaurs. The team heading up the discovery, led by Catherine Forster of the State University of New York at Stony Brook, named the creature Rahona ostromi, meaning Ostroms menace from the clouds.
Previous evidence mainly came from the primitive bird called Archaeoteryx. Upon its discovery, most have come to accept that birds are derived from tiny dinosaurs known as therapods. There are small, yet outspoken groups who still oppose the theory claiming that birds are far too old to be related to the therapods. They base their opinions on evidence showing that the therapods existence was roughly 115 million years ago, whereas the Archaeoteryx dates back to approximately 150 million years ago. However, Rahona ostromi is rather young on the evolutionary time line in comparisons, having its prime years between 65 to 70 million years ago. In addition, it has a multitude of elements which relate directly to the therapods.
The team found that the new specimen had had a claw on the second toe of its hind feet. This trait is shared by only a few who belong in the group of therapods named maniraptorans, which includes Deinonychus and Velociraptors. Like these dinosaurs, the toe in question is much thicker than that of other birds. In addition, Rahona ostromi bares a long, saurian tail.
Taking into account all of the similarities to a dinosaur which this creature has, it is still quite clearly a bird. Perhaps the creatures hips and legs are one of its strongest traits relating it to birds, although the bones are crucial as well. Its bones are light and hollow, and on its forearm bone there are six bumps which are almost definitely remains of where feathers used to be. A debate continues on whether they could fly or not. Scientists still ponder on whether the birds flew during those stages or if they tended to glide.
Forster and her colleagues have fit Rahona ostromi into the bird family tree right up near Archaeopteryx. The best explanation thus far for the huge difference in when they existed is attributed to Rahonas isolation on Madagascar. The team looks at the fossil as an evolutionary holdover of sorts. In any case, this discovery just about shuts the book on the debate over whether of not our modern day birds are descended from dinosaurs.
Kristin Leutwyler. Flyin Dinosaurs, Scientific American.