Tag Archives: Ancestor of vertebrates

Earliest signs of vertebrates’ ancestor?

Studies of DNA among living animals suggest that our own group, the vertebrates of the phylum Chordata, originated from a common ancestor that we share with echinoderms (sea urchins, star fish, sea cucumbers etc) and one of many worm-like phyla. This superphylum comprises the deuterostomes, but it is just one of several that encompass all animals and happens to be one of the smallest in terms of the number of living species that belong to it. We deuterostomes are vastly outnumbered by arthropods, nematodes, other worm-like creatures, molluscs, the rest of the animal kingdom and, of course, single-celled organisms, plants and fungi. Yet the DNA-based Circle of Life reveals that the deuterostome ancestral spoke originated early on in animal evolution.

The ‘Circle of Life’ as compiled by Cody Hinchliffe of the University of Michigan and 21 collaborators from the USA, and partly based on Fischetti, M. 2016. The circle of life. Scientific American, v. 314 (March 2016).

The ‘Circle of Life’ as compiled by Cody Hinchliffe of the University of Michigan and 21 collaborators from the USA, and partly based on Fischetti, M. 2016. The circle of life. Scientific American, v. 314 (March 2016).

The majority of animals of all kinds are blessed with a mouth separate from means of expelling waste products and can be divided into two similar halves, hence their name bilaterians. The earliest fossils judged to be of this kind date to about 580 to 600 Ma ago, in the Doushantuo Formation of southern China, all of them visible only using microscopes. A DNA-based molecular clock hints at around 900-1000 Ma for the emergence of all animal body plans known today. Now another important time marker has turned up, again in sediments showing exquisite fossil preservation from China (Han, J. et al. 2017. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature, v. 542, (online); doi: 10.1038/nature21072). The Chinese-British team of palaeontologists has found tiny, bag-like fossils preserved in phosphate, which have a mouth surrounded by folds and conical openings on either side of the body. They lived in limy muds on the sea bed now preserved as limestones at the base of the Cambrian System (541 Ma) and probably had a habit akin to worms in the most general sense. The authors sifted through 3 tonnes of rock to recover the fossils, rather than relying on a lucky hammer stroke.

Reconstruction of Saccorhytus coronarius from the lowest Cambrian of Shaanxi, China. (credit: Han et al 1917)

Reconstruction of Saccorhytus coronarius (diameter about 1 mm) from the lowest Cambrian of Shaanxi, China. (credit: Han et al 1917)

Not especially prepossessing, the fossils are said to show more affinity to deuterostomes than anything else and to be the earliest known fossil examples. Yet the world’s media pounced on them as the ‘earliest known human ancestors’, which is a bit rich as they could equally be the earliest sea urchins or may have led to several odd-looking fossils known only from the later Cambrian. It isn’t possible to say with any certainty that they lie on the path that led to chordates and thus ourselves. Of course, that would not raise headlines in newspapers of record, such as Britain’ Daily Telegraph, on the BBC News website or Fox News.  The authors are much more honest, claiming only that the Saccorhytus coronarius fossils are probably deuterostomes whose affinities and later descendants are obscure. Their most important conclusion is that the cradle of our branch of animals lay in deep water muds laid down around the Precambrian-Cambrian boundary, ideal for subtly varied small, flabby creatures behaving like worms.  Many more varieties are likely remain to be found in similar rocks of the late Precambrian and slightly younger Cambrian when they are studied painstakingly in microscopic detail. A start has been made, that’s all.

For more on early evolution see here and here