Fossilised Embryos – 500 Million Years Old

Evidence from fossilised embryos of worm-like creatures that lived 500 million years ago shows that embryos developed then in much the same way as their living relatives do today. The implications of this remarkable discovery, reported in this week’s issue of Nature, is that embryological processes that occur today must have been established very early on in the evolution of animals.

Because embryos are composed of tissues that decay away to nothing in an instant they are very rarely preserved in the fossil record. But Dr Phil Donoghue from the Department of Earth Sciences at Bristol University and his Chinese colleagues, sifted through 6,000 kg of rock from China to find just 100 exquisitely preserved embryos that are less than half a millimetre in length. They believe that the adult form of these embryonic ‘Markuelia’ were fairly large worm-like creatures with a mouth at one end, surrounded by a series of spines. They lived in the sea during the Cambrian, when animal life was just emerging.

The ‘tree of life’ is the single evolutionary ‘tree’ that links all plants, animals and microbes. When constructing the tree, one of the key questions that scientists are trying to answer is how the branches are connected to one another – how one animal is related to another – and what changes occurred to the growth of embryos that allowed those branching events to happen. But until now, despite the widespread availability of genetic data, almost everything that is understood about the evolutionary relationships between animals was based upon the classical approach of comparing the embryonic development of their living representatives.

These fossil embryos have enabled the team to determine that they represent creatures that are early relatives of both Arthropods (insects, lobsters, trilobites, etc) AND nematodes (parasitic worms that live in our guts, in soil, beer mats and everywhere else). Understanding this indicates that the changes in embryological processes that underlie animal evolution must have occurred long before these fossils were living 500 million years ago, and even before the earliest fossil remains of any animals that have been found in the geological record.

Ironically, it appears that the embryology of these fossils may now be better understood than their living counterparts which have not been studied very closely. The possibility of being able to describe the life-cycle of a creature that lived 500 million years ago is no longer in the realms of fantasy.