Evolution branches in a new direction

The quest to explain how modern humans emerged from the primordial slime has taken a new turn.

The place of humans and other backboned animals in the tree of life has been questioned by Flinders University researchers who claim a new evolutionary tree could overturn widely held beliefs that almost all vertebrates (backboned animals) are descended from placoderms (archaic armoured jawed fish).

The research, published this week in Oxford University Press journal Systematic Biology, challenges the widely-held idea that placoderms were the direct ancestors all other jawed vertebrates on Earth.  Instead, these armoured fish were merely a side branch in evolution – and humans and other vertebrates have more complex beginnings.

“So if placoderms were not our ancestors, what was?” asks Flinders University palaeontology researcher Benedict King. “Our study suggests that no particular group of known jawed vertebrates is ancestral to the others.

“Rather, the true jawed vertebrate ancestor probably combined features of bony fish (such as carp), cartilaginous fish (sharks and rays) and placoderms – in much the same way that the common ancestor of humans and chimpanzees was neither human nor chimp but a unique amalgam of both.”

The new model used by the Flinders team to reshape the tree of life combines traditional anatomical features along with other sources of information, such as the geological ages of the fossils and how much evolution they have undergone.

Recent fossil discoveries in China, some of the most ancient jawed vertebrate fossils known, combine placoderm and bony fish (or osteichthyan) features and may be the closest thing we know of to the true ancestors of jawed vertebrates.

The abrupt extinction of the diverse group of placoderms around 360 million years ago paved the way for carp, sharks and other modern groups to expand across all aquatic environments and start their evolutionary domination.

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The diagram illustrates how placoderms may be the primitive stock of all other jawed vertebrate descendants (see left) or instead a distinct side branch which left no living heirs, as suggested by the new research from Flinders University (right). Illustration by Benedict King and Dr Brian Choo, Flinders.

Recently, chinks have appeared in the popular view that all evolutionary roads lead back to placoderms, and this paper brings light to debates that have been ongoing within the palaeontological community for some time.

Imperial College London researcher Dr Martin Brazeau – an expert in vertebrate fossils from the Palaeozoic Era (approximately 540 to 250 million years ago) – says there has seen a considerable shift in how we understand placoderms in recent years.

“It was recently claimed (by me and others) that placoderms may be what we call a ‘grade’: an array of groups or species that form a succession of ever-closer relatives of modern jawed vertebrates.” says Dr Brazeau, who developed one of the first large-scale trees of life for early jawed vertebrate fossils.

“This is an ideal case for palaeontologists, because it helps us reconstruct detailed sequences of character evolution,” he says.

“However, the work by King and colleagues attacks this problem from a new angle, bringing detailed reinvestigations of existing data and—most significantly—new, cutting-edge methods. The combined effect of their work has upset the ‘new orthodoxy’ on placoderms.

“It helps resolve some of the problematic biological implications of recent work on placoderms—such as the origin of internal fertilisation. The new work will be sure to stimulate important debate about the placoderm problem and encourage new methodological approaches.”

Two researchers at the Institute of Vertebrate Paleontology and Paleonanthropology at the Chinese Academy of Sciences, who were involved in the discovery of these important early fossils, were collaborators in the Flinders study.

Unravelling the evolution of these early ancestors of jawed vertebrates will help to explain the diversity of backboned animals that populate the planet today, from fish, reptiles and birds to amphibians and mammals.

The paper, Bayesian Morphological Clock Methods Resurrect Placoderm Monophyly and Reveal Rapid Early Evolution in Jawed Vertebrates, was supported by vertebrate palaeontology and palaeoanthropology  researchers Dr Tuo Qiao and Professor Min Zhu from the Chinese Academy of Sciences in Beijing, Professor Mike Lee (South Australian Museum and Flinders University, Adelaide) and Strategic Professor John Long at Flinders University.

The Flinders University researchers received funding from the Australian Research Council.

Go to:

http://dx.doi.org/10.1093/sysbio/syw107 and also see http://theconversation.com/a-new-tree-of-life-challenges-our-ideas-about-vertebrate-evolution-68416

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