Fossils Bridge Gap in African Mammal Evolution

LONDON (Reuters) - Fossils discovered in Ethiopia's highlands are a missing piece in the puzzle of how African mammals evolved, a team of international scientists said on Wednesday.

Little is known about what happened to mammals between 24 million to 32 million years ago, when Africa and Arabia were still joined together in a single continent.

But the remains of ancestors of modern-day elephants and other animals, unearthed by the team of U.S. and Ethiopian scientists 27 million years on, provide some answers.

"We show that some of these very primitive forms continue to live through the missing years, and then during that period as well, some new forms evolved -- these would be the ancestors of modern elephants," said Dr John Kappelman, who headed the team.

The find included several types of proboscideans, distant relatives of elephants, and fossils from the arsinoithere, a rhinoceros-like creature that had two huge bony horns on its snout and was about 7 feet high at the shoulder.

"It continues to amaze me that we don't have more from this interval of time. We are talking about an enormous continent," said Kappelman, who is based at the University of Texas at Austin.

Scientists had thought arsinoithere had disappeared much earlier but the discovery showed it managed to survive through the missing years. The fossils from the new species found in Ethiopia are the largest, and at 27 million years old, the youngest discovered so far.

"If this animal was still alive today it would be the central attraction at the zoo," Tab Rasmussen, a paleontologist at Washington University in St Louis, Missouri who worked on the project, said in a statement.

Many of the major fossil finds in Ethiopia are from the Rift Valley. But Kappelman and colleagues in the United States and at Ethiopia's National Science Foundation (news - web sites) and Addis Ababa University concentrated on a different area in the northwestern part of the country.

Using high-resolution satellite images to scour a remote area where others had not looked before, his team found the remains in sedimentary rocks about 6,600 feet above sea level.

Here's a hint: it did not happen. There is no evidence of Neandertal dna in modern humans

Here's a hint: We have no adequate test for determining that. The only studies we have are of purely matrilineal mitochondrial DNA.

Mitonchondrial DNA, in other words, says nothing about who your daddy was. There is no sexual recombination at all.

Why is that a problem? Some few hundreds of neanderthals here and there throughout Europe mate with those pesky new gracile, high-brow types swarming into the region. They aren't a big part of the mix, but they merge in.

Let's pick a male. He marries a Cro-magnon female. His kids get half his nuclear genes. How much of his mtDNA do they have? Not one of them has any.

Let's pick a female neandethal who marries a Cro-magnon male. Again, her kids each get half of their nuclear DNA from her. How much of their mtDNA do they have? All of them have all of it. "Now we're cooking!" you say.

But let's be real and say half of her brood is male. None of her granchildren from them will have neanderthal mtDNA. It's only her direct female line that will carry the mtDNA. She's only one woman in a village with maybe ten women in it, and the Cro-magnons think she's butt-ugly.

Chances are her mtDNA line will die out, even if her nuclear genes are spreading all over the place.

Got That? The Complex Story of African Mammal Evolution 12/03/2003
The article by Jean-Jacques Jaeger in the Dec. 4 issue of Nature¹ is pretty upbeat about the evolutionary history of African mammals, but takes a bit of untangling to follow.
He begins confidently, “For some 40 million years [sic], the Afro-Arabian landmass existed in splendid isolation. A newly described fossil fauna from the end of that time provides a window [sic] on the evolution [sic] of the continent’s large mammals.” (He refers to a fossil group named the Chilga biota, found in the Ethiopian highlands by Kappelman et al., described in the same issue.²) Let’s take a look out said window and see how evolution has unfolded:

During most of the Cenozoic era, from the Cretaceous–Tertiary boundary 65 million years ago [sic] until roughly 24 million years ago [sic], Afro-Arabia was [sic] an island continent drifting steadily [sic] northwards towards Eurasia. Fossil mammals documenting this period are scarce and belong almost exclusively to endemic forms restricted to Afro-Arabia, such as proboscideans, hyraxes and elephant-shrews. But by around 24 million years ago [sic], a permanent land bridge had formed between the two landmasses. A burst of faunal interchange followed: many Eurasian mammals, such as rhinos and ruminants, dispersed into Africa, and some Afro-Arabian mammals, such as elephants, migrated in the opposite direction. (Emphasis added in all quotations.)

That forms the plot line, but there are problems. The Chilga specimens he describes seem to fit the story, but there are puzzles among the bones:

Among the proboscideans recorded are primitive [sic] forms such as Palaeomastodon and Phiomia (also known from older deposits in Egypt). But there are also representatives of modern families, for example taxa such as Gomphotherium, the earliest proboscidean on the branch leading to extant elephants. Another surprise is the oldest occurrence of deinotheres, peculiar proboscideans with downward-curved lower tusks, which were previously recorded only from rocks younger than 24 million years old [sic]. The new species of deinothere displays molars that are more ‘bunodont’ in form (that is, made of several distinct cusplets arranged in transverse crests) than its descendant, whose molars display plain transverse crests. This discovery seems to rule out the possibility that deinotheres are derived from an ancestor bearing plain, transverse-crested molars, as was formerly supposed, and provides new evidence about proboscidean evolution [sic].

Jaeger bemoans the scarcity of the fossil record for this period, but claims, “Nonetheless, considerable information has been inferred from the evidence we do have.” He talks about how systematists have grouped the African fauna into a superorder Afrotheria based on fossil and molecular evidence. Though “African mammalian faunas are dominated by these endemic forms,” a few other groups did get over to the big island somehow, including our alleged remote ancestors, the catarrhine primates, fathers of hominoids. These “newcomers” went through “rapid evolution” on the landmass, he claims.
Even though the Chilga fossils are supposed to pre-date the land bridge, Jaeger says, “The Chilga mammals also yield insights into the dynamics of the faunal interchange between Afro-Arabia and Eurasia.” How is that possible? By seeing what pre-existed before the interchange, he feels it is possible to document that “the ensuing ecological competition ended with winners and losers.” I.e., some animals were destined to fall in numbers, others to multiply and diversify.
The Chilga fossils do leave a few research items for paleontologists:

Finally, the discoveries of Kappelman et al. highlight two other palaeobiological issues. First, on northern continents glaciation caused a significant cooling around 33 million years ago [sic], which resulted in numerous extinctions [sic] among mammalian communities. From these new data, however, it seems that large Afro-Arabian herbivores were not affected, either at that time or later, implying that the climatic changes were less severe on southern continents. Second, the fossil record of the Afro-Arabian continent is not only scanty but also largely concentrated on the northern edge. This has led to the proposal that other groups of mammals existed in Afro-Arabia during its period of isolation, but that they were restricted to more southern latitudes. However, the Chilga mammal community is rather like that found at Fayum in Egypt, which is some five million years older [sic], providing hints that there was little provinciality among Afro-Arabian mammals at that time. As yet, though, we have unveiled [sic] only a few of the secrets of mammal evolution on the Afro-Arabian continent. Many more surprising discoveries are to be expected.

Got that?

¹Jean-Jacques Jaeger, “Mammalian evolution: Isolationist tendencies,” Nature 426, 509 - 511 (04 December 2003); doi:10.1038/426509a.
²Kappelman et al., “Oligocene mammals from Ethiopia and faunal exchange between Afro-Arabia and Eurasia,” Nature 426, 549 - 552 (04 December 2003); doi:10.1038/nature02102.

We’re going to give our readers an opportunity to go baloney detecting on this story before we have at it. Hint: separate data from storytelling, and look for the damaging admissions.

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