Darwin 200: worms & vertebrates

All this week we're celebrating Darwin and the advances in evolutionary theory that have built upon his work.

I asked Peter Holland of Oxford's Department of Zoology about early vertebrates, worms and us:

OxSciBlog: How has genomic science changed how we think about evolution?
Peter Holland: Genomics hasn’t changed the way we think about evolution at a fundamental level – I mean, even before genomics we knew that evolution is a fact consistent with all biological knowledge, that all life on earth diversified from one origin, and that the dominant forces underpinning evolution are mutation and selection. But what genomics has done is added colour and detail to that picture.

For example, sequencing of genomes has given us new insights into the different types of mutation, not only small changes to individual genes, but big mutations that copy whole sets of neighbouring genes in tandem, or duplicate entire genomes.

And we should not forget the technological advances that genomics has driven, such as faster, cheaper DNA sequencing. This now enables researchers to deduce the past history of life on earth using hundreds of genes at a time, not just one or two, with some major new insights into the true tree of life.

OSB: How far back can we trace the basic body plan of vertebrates?
PH: All vertebrates share the same fundamental body plan, with a complex head built from moving ‘neural crest’ cells, a subdivided brain, segmented blocks of muscle and a skeleton of some sort. Only vertebrates have all these features together, and even the oldest vertebrate fossils show signs of them.

Interestingly, when we look at our closest non-vertebrate relatives (amphioxus and sea squirts) we find some of these features in rudimentary form, implying that the extinct vertebrate ancestors also had these rudimentary structures.

So when the first vertebrates evolved, perhaps 550 million years ago, evolution just elaborated on what was already present, by adding some new cell types such as bone, cartilage and an adaptive immune system.

OSB: Why, in terms of evolution, are genome duplication events important?
PH: In 1882, the satirical magazine Punch published a cartoon about Darwin entitled ‘Man is but a worm’. We now know that actually ‘Man is but four worms’, because at the origin of vertebrates the whole genome – every gene – was duplicated twice. Many of the extra genes were lost after this event, but vertebrates still retain more genes than most invertebrates.

It is tempting to suggest that these extra genes were recruited for new roles, allowing new tissue and cell types to evolve, and in fact there is evidence to support this hypothesis.

However, this does not mean that genome duplications are always associated with adaptation – for example, the squid and octopus lineage evolved anatomical complexity without genome duplication, while teleost fish experienced a genome duplication without obvious increase in complexity.

OSB: What major questions about the evolution of early vertebrates remain to be answered?
PH: We don’t have a good handle on the timeframe – we need more fossils to give us better dates. We also don’t fully understand how new genes gain new roles, and this is notoriously difficult to study. And finally it has been difficult to work out how lampreys and hagfish fit into the history  – did they branch off before the two genome duplications, after, or in between? This last point has spawned many heated discussions!  

Peter Holland is Linacre Professor of Zoology and Associate Head of the Department of Zoology at Oxford.