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article imageNew study claims humans evolved from primitive species of shark

By JohnThomas Didymus     Jun 14, 2012 in Science
A new study says all jawed vertebrates, including humans, evolved from a species of shark that lived about 300 million years ago. Scientists came to the conclusion after analysis of a 290-million-year-old fossil braincase.
According to the Daily Mail, scientists say all jawed vertebrates, including humans, evolved from the primitive shark called Acanthodes bronni.
Researchers came to the conclusion after they conducted a two-year analysis of a 290-million-year-old fossil braincase of the shark species.
Researchers say the braincase showed the shark belongs to the same group as modern animals called gnathostomes -- animals with jaws. Gnathostomes include several extant vertebrates including fish, birds, reptiles, mammals, including human beings.
The Telegraph explains the word "Acanthodes" is derived from Greek for "spiny."
The acanthodes sharks lived before the evolutionary divergence of the earliest cartilaginous sharks and the first bony fishes. Bony fishes eventually gave rise to human beings, evolutionary scientists say.
Cartilaginous fish include sharks and rays. Scientists say they diverged from bony fishes about 420 million years ago.
Acanthodes were relatively large compared with other known "spiny" sharks. They measured about a foot in length, and had gills, large eyes and fed on plankton.
Professor Michael Coates, biologist at the University of Chicago, said, "Unexpectedly, Acanthodes turns out to be the best view we have of conditions in the last common ancestor of bony fishes and sharks. Our work is telling us the earliest bony fishes looked pretty much like sharks, and not vice versa. What we might think of as shark space is, in fact, general modern jawed vertebrate space."
Scientists say the acanthodians became extinct about 250 million years ago. They left behind too scanty fossil records to allow scientists to make a detailed reconstruction of what they looked like. IB Times reports that there had been uncertainties about correct classification of Paleozoic era acanthodians because the evidence from fossilized scales and fin spines gave confusing evidence on whether they should be classified as sharks or bony fish.
But after scientists acquired new data about what the earliest sharks might have looked like, they conducted a second analysis of the evidence from the braincase of Acanthodes bronni, the best preserved of the known species.
According to Professor Coates, "We want to explore braincases if possible, because they are exceptionally rich sources of anatomical information. They are much better than scales, teeth or fin spines, which, on their own, tend to deliver a confusing signal of evolutionary relationships."
The researchers say that recent analysis of samples and scans of the skulls of early sharks and bony fishes led to unexpected conclusions about the evolutionary history of jawed vertebrates. IB Times reports the study began with fresh analysis of the braincase of Acanthodes bronni. Co-author Samuel Davis created detailed latex molds that revealed details of the inner and outer parts of the skull. The information was used to reconstruct cranial and jaw anatomy, and the structure of sensory, circulatory and respiratory systems.
Coates said: "For the first time, we could look inside the head of Acanthodes, and describe it within this whole new context. The more we looked at it, the more similarities we found with sharks."
The discovery that the evolutionary history of all jawed vertebrates converge on ancient species of sharks may give insight into the evolutionary transition from jawless to jawed vertebrate animals. According to Coates, "It helps to answer the basic question of what is primitive about a shark. And, at last, we are getting a better handle on primitive conditions for jawed vertebrates as a whole."
According to environmental biologist Dr Maureen Kearney of the National Science Foundation in the US, the new study shows "important evolutionary transitions in the history of life, providing a new window into the sequence of evolutionary changes during early vertebrate evolution."
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