origin of humans; a place for Sahelanthropus (Origins)

by David Turell , Tuesday, July 09, 2024, 21:42 (100 days ago) @ David Turell

An ancestor from seven million years ago:

https://www.newscientist.com/article/2438905-when-did-human-ancestors-start-walking-on-...

"The period between 7 million and 4 million years ago is a bit of a nebulous phase in the story of human evolution. There are basically four data points: Sahelanthropus tchadensis from 7 million years ago, Orrorin tugenensis from 6 million years ago and the two species of Ardipithecus from 5.6 million and 4.4 million years ago. Each is known from a handful of incomplete fossils. For a period of 3 million years, that’s not much. For comparison, there are dozens of Neanderthal sites from the past 500,000 years.

***

"Despite being the oldest known hominin, Sahelanthropus is a relatively recent discovery, first described in 2002 by a group of researchers. The remains were found in the deserts of Chad, which is in north-central Africa, a long way from eastern countries of the continent like Ethiopia and Tanzania that had yielded many famous fossils.

"The main find was a skull, which was named Toumaï... writer Jeff Hecht said it didn’t resemble any modern great ape: “Although its body and brain were the size of a modern chimp’s, its face was quite different, with large brow ridges and much smaller canine teeth.” The researchers also found some teeth and bits of jawbone.

"The French researchers who described Sahelanthropus argued it was bipedal. This was based mainly on the base of the skull and how it apparently fitted onto the spine: it looked like the skull sat directly atop the spinal column, as opposed to being at an angle like in an ape skeleton. It was an intriguing argument – but far from conclusive.

***

"In 2018, the pair tried to present their own analyses of the femur at a conference at the University of Poitiers, but they were blocked by the organisers....

"The following year, Macchiarelli and Bergeret-Medina submitted a paper about the femur... which was finally published in November 2020. The key point was that the Sahelanthropus femur was curved. This is typical of a great ape like a chimpanzee, and not what you’d expect of an upright-walking hominin. Our leg bones are straight because they need to act like pillars supporting the entire weight of our bodies. I consulted two palaeoanthropologists,.. and they both agreed: Sahelanthropus didn’t look like a biped.

"However, the original Poitiers research team, after years of silence, decided to start talking....

"Guy and his colleagues highlighted a number of features of the femur that they say indicate bipedality. For instance, thicker regions along the shaft of the bone correspond to those seen in modern humans and are different from those in great apes. There was also “a rough surface at the top of the femur where the buttock muscles attach”.

***

"The overall message is that the few pieces we have of Sahelanthropus’s limbs don’t show strong evidence of habitual bipedal walking. “It’s generally indistinguishable from the African apes,” says Zanolli.

***

"For now, we can’t even be sure that bipedality evolved in Africa. It’s tempting to think so, because the oldest bipedal hominins we know of are African, even if you discount Sahelanthropus. A study published in May combined the locations of known hominin and ape fossils and their suspected relationships, and concluded that the group that includes both chimpanzees and hominins probably originated in north-central Africa.

"A paper from March suggested that the last common ancestor of hominins and other African apes lived in Eurasia, but that a dramatic event separated the population into two, which then evolved independently.

"What dramatic event? Why, the Zanclean Megaflood of course. If you don’t know, there was a period between about 6 million and 5.3 million years ago when the Mediterranean almost entirely dried out. The Strait of Gibraltar, which connects the Mediterranean to the wider Atlantic, closed – and the sea gradually evaporated, leaving hypersaline lakes. This was the Messinian Salinity Crisis.

"Then, around 5.3 million years ago, the Strait of Gibraltar reopened – and the waters of the Atlantic came rushing in. One reconstruction estimated that it took “from a few months to two years” to refill the Mediterranean basin, which, if not an apocalyptic mega-tsunami, is still pretty fast.

"Supposedly, this Zanclean Megaflood cut off one population of apes/hominins on the Arabian peninsula, while others were able to reach Africa – creating an evolutionary split. In May, a separate group went further and linked the closure and reopening of the Mediterranean to changes in the behaviour of the Pacific tectonic plate.

"You may be able to tell that I’m unconvinced by all of this. It seems to me there are far too many intermediate steps from the Pacific tectonic plate and the Zanclean Megaflood to hominins habitually walking upright, and we can’t be sure about any of them. We don’t even know the timing of the origin of bipedality. If either Sahelanthropus or Orrorin was bipedal, the behaviour evolved well before the megaflood.

"So, much as I want to link the origin of hominins to the biggest flood of the past 10 million years, I think we probably ought to find some more fossils first."

Comment: a low fossil count in this period may mean very small populations of these forms. We obviously evolved from some ape form. The Mediterranean flood is a fascinating event to add to the confusion.