Human origins traced to worm fossil in Canada

March 5th, 2012 in Other Sciences / Archaeology & Fossils
Pikaia gracilens. Image credit J.B. Caron


Pikaia gracilens. Image credit J.B. Caron

(PhysOrg.com) -- Most primitive known vertebrate and therefore the ancestor of all descendant vertebrates, including humans, discovered.

Researchers from the University of Cambridge, University of Toronto and the Royal Ontario Museum (ROM) have confirmed that a 505 million-year-old creature, found only in the Burgess Shale fossil beds in Canada’s Yoho National Park, is the most primitive known vertebrate and therefore the ancestor of all descendant vertebrates, including humans.

 The research team’s analysis proves the extinct Pikaia gracilens is the most primitive member of the chordate family, the group of animals that today includes fish, amphibians, birds, reptiles and mammals. The study is based on the analysis of 114 specimens and was published yesterday, March 5, in the British scientific journal Biological Reviews.

Pikaia was first described, on the basis of only a few specimens, by American palaeontologist Charles Doolittle Walcott in 1911 as a possible annelid worm, a group that includes today’s leeches and earthworms. However, scientists have long speculated that Pikaia was a chordate because it appeared to have a very primitive notochord – a flexible rod found in the embryos of all chordates – which goes on to make up part of the backbone in vertebrates.

“The discovery of myomeres is the smoking gun that we have long been seeking,” said the study’s lead author, Professor Simon Conway Morris of the University of Cambridge. “Now with  myomeres, a nerve chord, a notochord and a vascular system all identified, this study clearly places Pikaia as the planet’s most primitive chordate. So, next time we put the family photograph on the mantle-piece, there in the background will be Pikaia.”

“Our analysis provides evidence that Pikaia indeed had a notochord,” said co-author, Jean-Bernard Caron, an assistant professor of ecology and evolutionary biology at the University of Toronto and curator of invertebrate palaeontology at the ROM. A nerve cord and vascular system were also identified in the study.  “But the real excitement was finding extensive myomeres, the blocks of skeletal muscle tissue that are characteristic of chordates.”

Averaging about five centimetres in length, Pikaia was a sideways-flattened, somewhat eel-like animal. The flattened body is divided into a series of segmented muscle blocks seen as S-shaped lines that lie on either side of the notochord which runs along most, if not all of the body length. It likely swam above the sea floor by moving its body in a series of side-to-side curves.

The Burgess Shale is famous for its weird and wonderful fossils of marine organisms. The site provides vital information about evolution during the Cambrian explosion, a period over half a billion years ago that was characterized by the appearance of a vast diversity of animals over a short period of time.

The study examined 114 Pikaia fossils using a range of imagery techniques, including scanning electron microscopy, to reveal fine details. The bulk of the specimens are held in trust for Parks Canada at the ROM while nearly all the remainder are housed in the National Museum of Natural History, part of the Smithsonian Institution in Washington, DC.

“It’s very humbling to know that swans, snakes, bears, zebras and, incredibly, humans all share a deep history with this tiny creature no longer than my thumb,” said Caron.

“Fossils of primitive chordates are incredibly rare. With no backbones or other mineralized elements, Pikaia would stand no chance of preservation in normal conditions outside exceptional sites like the Burgess Shale. We hope that, with continuing explorations and field work studies there, other species will be discovered allowing us to refine our understanding of the early history of our own group.”

The confirmation of Pikaia as a chordate is the latest in a recent string of Burgess Shale discoveries. In November 2011, evidence of fossilized tracks of a large predator known as Tegopelte were published, and in January 2012 a bizarre tulip-shaped creature named Siphusauctum was described for the very first time.

More information: Pikaia gracilens Walcott, a stem-group chordate from the Middle Cambrian of British Columbia, Article first published online: 4 MAR 2012. DOI: 10.1111/j.1469-185X.2012.00220.x

Abstract
The Middle Cambrian Pikaia gracilens (Walcott) has an iconic position as a Cambrian chordate, but until now no detailed description has been available. Here on the basis of the 114 available specimens we review its anatomy, confirm its place in the chordates and explore with varying degrees of confidence its relationships to both extant and extinct chordates and other deuterostomes. The body of Pikaia is fusiform, laterally compressed and possesses about 100 myomeres. The head is small, bilobed and bears two narrow tentacles. There is no evidence for eyes. Apart from a thin dorsal fin (without finrays) and a series of at least nine bilaterally arranged appendages with possible pharyngeal pores at the anterior end, there are no other external features. In addition to the musculature the internal anatomy includes an alimentary canal, the anterior of which forms a prominent lenticular unit that is almost invariably preserved in positive relief. The cavity is interpreted as pharyngeal, implying that the mouth itself was almost terminal. The posterior extension of the gut is unclear although the anus appears to have been terminal. The most prominent internal structure is a reflectively preserved unit, possibly hollow, termed here the dorsal organ. Although formerly interpreted as a notochord its position and size make this less likely. Its original function remains uncertain, but it could have formed a storage organ. Ventral to the dorsal organ a narrower strand of tissue is interpreted as representing the nerve chord and notochord. In addition to these structures, there is also evidence for a vascular system, including a ventral blood vessel.
The position of Pikaia in the chordates is largely based on the presence of sigmoidal myomeres, and the more tentative identification of a notochord. In many other respects, Pikaia differs from the expected nature of primitive chordates, especially as revealed in amphioxus and the Cambrian record (including Cathaymyrus, Haikouichthys, Metaspriggina, Myllokunmingia, and Zhongxiniscus). Whilst the possibility that Pikaia is simply convergent on the chordates cannot be dismissed, we prefer to build a scenario that regards Pikaia as the most stem-ward of the chordates with links to the phylogenetically controversial yunnanozoans. This hypothesis has implications for the evolution of the myomeres, notochord and gills. Finally, the wealth of material of Pikaia indicates that, although by definition there must be some sort of taphonomic imprint, the consistency of preservational details allows a reliable reconstruction of the anatomy and does not significantly erode phylogenetically relevant characters.

Provided by University of Cambridge

"Human origins traced to worm fossil in Canada." March 5th, 2012. http://phys.org/news/2012-03-human-worm-fossil-canada.html