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Afradapis longicristatus fossil discovery

An early adapid species that has become newly known to science as Afradapis longicristatus due to a initial fossil discovery made in Egypt in 2001, and which has been dated as having lived some 37 million years ago, is described as part of an evolutionary study in the prestigious science journal Nature (October 22, 2009) by a team led by Erik R. Seiffert, Ph.D., Assistant Professor in the Department of Anatomical Sciences at Stony Brook University in New York state.

The article in Nature was entitled “Convergent evolution of anthropoid-like adaptations in Eocene adapiform primates.” In addition to Dr. Seiffert, co-authors of the study include: Jonathan M. G. Perry, Department of Anatomy, Midwestern University; Elwyn L. Simons, Division of Fossil Primates, Duke Lemur Center, Duke University; and Doug M. Boyer, Department of Ecology & Evolution, Stony Brook University.

Afradapis longicristatus features strongly in their study and is thought to be closely related to Darwinius masillae another early adapid species that has been dated as living some 47 million years ago.

In the sciences parsimony is valued in terms of a preference for the least complex explanation for any observation. The principle that "when you have two competing theories that make exactly the same predictions, the simpler one is the better" is generally regarded as good when judging hypotheses.

The Sieffert led team's analysis used the generally accepted scientific method called parsimony evidence in comparing 360 anatomical features of 117 living and extinct primates in their efforts to arrive at an hypothetical phylogenetic tree or evolutionary tree for the primate family.

In biological and evolutionary science a phylogenetic tree or evolutionary tree is a tree showing the evolutionary relationships among various biological species or other entities that are believed to have a common ancestor.

Darwin's evolutionary theory tree of life sketch of 1837Perhaps the earliest example of a phylogenetic tree or evolutionary tree was actually devised by Charles Darwin. His approach can perhaps be illustrated by this famous Tree of Life sketch from his Notebook B dating from 1837-8:-

Charles Darwin's early evolutionary theory insight of how a branching tree-like genus of related species might originate by divergence from a starting point (1) to effectively establish related species at such notional points as A, B, C and D.

There is an accompanying text annotation that reads:-

I think

Case must be that one generation then should be as many living as now. To do this & to have many species in same genus (as is) requires extinction.

Thus between A & B immense gap of relation. C & B the finest gradation, B & D rather greater distinction. Thus genera would be formed. — bearing relation (page 36 ends - page 37 begins) to ancient types with several extinct forms.

From Darwin's notebook B now stored in Cambridge University library.

In May, 2009, the conclusions of scientific researches based on the the discovery of the Darwinius masillae / Ida fossil were trimphantly announced to the world depicting Darwinius masillae / Ida, despite having many characteristics linking it with the adapid family and hence with modern lemurs, as something of a "missing link" fossil in terms of the origin of the higher primates and possibly with anthropoid, hominid and human ancestry.

picture of the Darwinius masillae fossil
Picture of the Darwinius masillae /Ida fossil

Due to the high-profile announcements in relation to Darwinius masillae in May, 2009, and to a possible close relationship between Darwinius and the newly announced Afradapis longicristatus, the team took up an opportunity to place a distinct focus on the Darwinius masillae and Afradapis longicristatus fossils. The results of their study indicate strongly that both Darwinius masillae and Afradapis longicristatus appear to be members of a group more closely related to lemurs and lorises than to monkeys, apes and people, and that both have left no known modern descendants.

“Our analysis is the first to incorporate evidence from all the key players in the anthropoid origins debate, that is all of the fossil species that have been proposed as possible early anthropoids, including a large sampling of adapiform primates,” said Dr. Seiffert. “The adapiform lineage that includes Darwinius and Afradapis has been particularly controversial, and we are finding new evidence that allows us to be increasingly confident that the anatomical features that these adapiforms share with anthropoids are due to convergent evolution and not common ancestry.”

In this tree, adapiform primates like Darwinius and Afradapis are not placed close to higher primates, but rather are situated as closer relatives of the living lemurs and lorises, which are "prosimian" primates.

To quote Dr. Sieffert
"We compiled a large dataset of anatomical observations, made across 117 living and extinct primates, including all of the fossil primates that have been proposed as possible early members of the anthropoid group...
...We used a computer program to find the primate family tree that provides the simplest explanation for the distribution of these traits. In that tree, adapiform primates like Darwinius and Afadapis are not placed close to higher primates, but are situated as closer relatives of the living lemurs and lorises."

To quote Dr. Sieffert again
"Our analysis provides no support for the claim that Darwinius is a link in the origin of higher primates, and instead indicates that, if anything, Darwinius is more relevant for our understanding of the origins of lemurs and lorises - which are our most distant primate relatives."
The team led by Dr. Seiffert record the surprising discovery that the first African primates to evolve large body size and tooth and jaw features like those of the living catarrhine anthropoids (that is, the Old World monkeys, apes, and humans) arose from within this distantly related prosimian group (adapiforms) that includes Afradapis.

The study accepts that such early promisians could have attained mature weights of 20 pounds (9 kilos) in contrast to the 1-2 pounds mature weight they attribute to the contemporaneous early ancestors of all later anthropoids.

"The adapiform lineage that includes Afradapis appears to have gone extinct near the Eocene-Oligocene boundary, around 34 million years ago, whereas multiple anthropoid lineages persisted into the Oligocene in Africa and continued to diversify up to the present day. Thus at a time when the early ancestors of all later anthropoids only weighed in at 1-to-2 pounds, had relatively small brains, and were dependent on a diet of fruit and insects, they were living alongside prosimian species that were in some cases 10 times larger, and had teeth, jaws, and feeding habits that were much more like their anthropoid descendants, which don't appear in the fossil record in Africa until many millions of years later."

"The extinction of these larger adapiform prosimians might have played an important role in freeing up feeding niches that anthropoids would ultimately come to occupy."

picture of the Afradapis longicristatus teeth
Picture of the Afradapis longicristatus teeth

The team regard the evolution of particular tooth and jaw features, (i.e. monkey-like jaw, spatula shaped teeth), in Afradapis longicristatus as being an example of so-called "convergent evolution" only arising from similarities developed simply because the same eating habits were shared by species that were nevertheless divergent in other ways. Generally speaking convergent evolution is a process in which organisms and animals become similar in shape or structure, in response to similar environmental conditions, despite their evolutionary lineages being different.

There is a growing body of evidence that indicates that convergent evolution was a common phenomenon in early primate evolution.

It seems that the common ancestry theory of adapiforms linking them more closely to higher primates than lemurs hinges on features such as fusion of the two halves of the jaw, reduction and loss of the first frew premolar teeth, and the presence of front teeth (incisors) that are shaped like a spatula. However, this article in Nature points out that the study of Afradapis shows the fusion of the two halves of the jaw clearly evolved convergently in adapiforms and anthropoids, as even the earliest anthropoids have unfused mandibles.

“Incisor teeth that are shaped like a spatula might have been present in the last common ancestor or all primates, and so would not specifically support a link between adapiforms and anthropoids,” said Dr. Seiffert. “Our analysis also indicates that the reduction and/or loss of the first few premolars must have evolved convergently in adapiforms and anthropoids because of some of Afradapis’ close relatives retain a full complement of four premolars on each side of the jaw, as in many other early mammalian relatives of primates.”

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Dr Seiffert holds that the Darwinius fossil is somewhat unreliable because its skull is crushed and its ankle damaged depriving science of two key areas of potential understanding of early primate evolution and has said that "when it comes to the key anatomical features that primate palaeontologists so often depend on, Darwinius is surprisingly uninformative".

Chris Beard, curator of vertebrate palaeontology at the Carnegie Museum of Natural History in Pittsburgh, U.S., who was initially openly sceptical at the suggestion that Ida was a missing link commented that “[the new analysis] confirms my views. Ida is about as far away from the anthropoid lineage as one can get and still be a primate.”

Jørn H. Hurum of the Norwegian Natural History Museum / University of Oslo (who oversaw the Darwinius masillae / Ida fossil study), and some of his colleagues, went on the record in declining to fully accept the Sieffert team's conclusions.

Dr Hurum has pointed out that the Afradapis fossil is incomplete, mostly made up of a lower jaw,
"How can a fragmented lower jaw have more useful information than a complete skeleton?"
and has said that:
"We expected a challenge like this and it's interesting it has taken five months for the first attack to come. What we claim about Ida is really quite controversial. Seiffert and his team claim Darwinius didn't have much anatomical detail to study because it is so crushed, but none of the authors have ever seen the original specimen. She's not that crushed, there's a lot of information in the fossil. We really trust and stand by our interpretation."
Dr. Philip Gingerich, a palaeontologist-from Princeton University in the US who worked with the Norwegian on Ida, described the Sieffert team's assertions as 'puzzling', adding that the creature was almost certainly part of the lineage that led to monkeys, apes and humans.

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Essay on Population
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Thomas Henry Huxley
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