by Matthew Cobb
Cats and humans look very different. Or do they? 32-day old embryos of the two species look pretty identical, as shown here (guess which is the cat! Answer at the end):
Why do they look so similar? This is one of the oddest riddles in biology, which has now been given a new answer following the publication of two articles in Nature today. To understand why this is so important, we need to go back nearly 200 years.
In 1828, Karl von Baer – the first man to observe the human egg – studied a number of embryos of different species and noted that the common features of large groupings of organisms, such as vertebrates, develop before the more specific characters. So, for example, human embryos have tails, and they also have folded structures around their neck that are similar to those seen in fish embryos, and which turn into gills in our water-dwelling relatives.
At the end of the 19th century, the German embryologist Ernst Haeckel turned this observation into an evolutionary “law”, in which he suggested that each organism, as it grows, goes through its evolutionary past. He put this in a fancy phrase: “Ontogeny recapitulates phylogeny”, and provided some neat drawings (these are from Wikipedia):
Although this might sound clever, it is not actually true (and it has been suggested that Haeckel’s drawings were not as accurate as they should have been!). Human embryos do not have gills – the embryonic structures become our jaws, not gills. Furthermore, Haeckel’s idea was based on a linear view of evolution that is plain wrong. Species do not evolve in a straight line, but a complex branched pattern. We share a common ancestor with a cat, but our lineages have been going their own way – and evolving differently – ever since.
But even if Haeckel was wrong, it doesn’t get away from the bizarre fact that human embryos have tails. What’s going on there? It turns out that the situation is more complex than Haeckel realised. At the earliest and latest stages of embryonic development, organisms within a given group look very different. But in a middle phase, they all tend to look pretty much the same. This effect is at its strongest in the different animal phyla – vertebrates, arthropods and so on – and is referred to as the “phylotypic” period, because each phylum has a typical common embryonic form which species in that phylum adopt, before turning into their final, species-specific shape.
This “hourglass” shape of embryonic development – specific/common/specific – has long intrigued biologists, but some people felt it might not be true, as it was based primarily on observations of physical similarity, not on the underlying genetic processes. Nature has just published two studies of very different model systems – the fruitfly Drosophila and the zebrafish – that cast a new light on the genetics of the “hourglass”, showing that the oldest genes are involved in the middle, “phylotypic” period.
The study on Drosophila – carried out by a team led by researchers from the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, Duke University and the University of Manchester – looked at the genes that are expressed during the embryonic development of fruitfly eggs from six Drosophila species, some of which have been separate for 40 million years. Even within these apparently-identical species (one fruitfly looks pretty much like another, unless you are an entomologist – or a fly), there are big differences in the genes that are expressed early and late in development but, just as predicted by the hourglass model, each species tends to express the same genes during the middle, “phylotypic” period.
As you might expect, these middle genes are involved in important processes that are common to all the Drosophila species, such as synthesizing biomolecules, organizing chromosomes and above all controlling anatomical development. Genes that are not involved in development tended not to follow the “hourglass”.
In a partner article, researchers from the Max Planck Institute for Evolutionary Biology and Ruđer Bošković Institute looked at the genes that are activated in 60 different stages of the zebrafish, from the embryo on into the adult. As with Drosophila, they found that the oldest genes were expressed during the middle, “phylotypic” period of embryonic development. Younger, more specific, genes tended to be expressed in the earlier and later phases, with the intriguing exception of the oldest animals, which once again tended to express evolutionarily ancient genes.
Fascinating as these studies are, they only answer half the question. We now know how this “phylotypic” middle phase of the “hourglass” is produced – through the expression of ancient genes. But we do not know why.
The Drosophila group found evidence that natural selection in the form of selective constraint is acting on these patterns of gene expression, perhaps related to the fact that there is a best way of assembling the shared aspects of each body shape, constraining innovation and resulting in the conservation of ancient, highly effective genes involved in development.
The zebrafish group take a slightly different tack and refer to Darwin’s suggestion that environmental influences might be strongest at the earliest and latest phases of development, meaning that in the middle, phylotypic phase, the embryo is less “visible” to adaptation by natural selection, leading to a lack of evolutionary innovation. This would also explain the fact that very old fish express very ancient genes – once an organism is past reproductive age, it generally cannot be “seen” by natural selection.
These two papers shed a new light on one of the most bizarre questions in biology – why do so many embryos look alike? They show that although ontogeny does not recapitulate phylogeny, phylogeny is involved in ontogeny – the way we grow does shed light on our evolutionary past. The “hourglass” is real at the visible and molecular level, and the genes that specify the middle, more general, shape of an organism are some of the oldest in each species’ genome.
Now we need to know why, and above all why did you and I have a tail when we were only a few weeks old?
In commenting on an earlier draft of this post, Jerry responded to this final point this way:
“I think we know the answer to that question–at least sort of, but you imply that it’s a complete mystery. Clearly our development evolved from an ancestral developmental plan, that that ancestor had a tail, gills, etc. We retain that plan (yes, we’re not sure why, but probably because changing it would have screwed up everything downstream). Don’t you think you should give the readers at least a BIT of explanation about this stuff? It is, after all, the question with which you begin!”
Well sorry, folks, but Jerry’s pearls of wisdom are as much as you’re going to get on that point. Come up with your own tales about tails in the comments below!
Answer: The cat is on the left. Or maybe it’s the right.
[Edit: There's a nice summary of these papers over at Discovery, together with some useful quotes from the authors.]
Alex T. Kalinka, Karolina M. Varga, Dave T. Gerrard, Stephan Preibisch, David L. Corcoran, Julia Jarrells, Uwe Ohler, Casey M. Bergman & Pavel Tomancak (2010). Gene expression divergence recapitulates the developmental hourglass model. Nature 468, 811–814.