Tuesday, November 11, 2008

Evolutionary Novelty: Hair

Mammals have hair but no other animals do. As such, hair is a clear evolutionary novelty, present in one group but absent in all others. In my macroevolution course (EEMB 102), I use hair as a clear character that can be used in phylogenetics. Hair groups all mammals to the exclusion of other organisms. In systematics jargon, hair is therefore a “synapomorphy”, grouping mammals together.

We can map the trait of hair on a family tree of animals. From this perspective, we can infer that the ancestor of all mammals very likely had hair, but that the ancestor of sauropods (birds, reptiles, and mammals) lacked hair. Therefore, hair originated prior to the common ancestor of all mammals.

Figure 1 – Hair originated before mammals, but after the common ancestor of birds, reptiles and mammals. Grey ellipse (hard to see except as a broken branch, I'd fix it but I'm too lazy) is the origin of hair keratin protein.

So where did hair come from - how did this evolutionary novelty evolve? A new paper by Eckhart et al in PNAS [link] provides evidence that the building blocks of hair pre-date the origin of hair itself. Namely, they found alpha-keratin (“hair keratin”) proteins are encoded in the genomes of chickens and the green anole lizard. In the green anole they studied, ‘hair keratin’ proteins were used in claws.

Just ten years ago, results like this clarifying the molecular components of trait evolution were rare, but they have become common now that genome sequences are available for many species. Before we had some idea of gene function, and before genome sequencing, scientists could only examine one level of biological organization – the trait (hair in this case). And that could only get science so far. In the case of hair, it mainly got science as far as Figure 1, which leads to the inference that hair evolved a bit before the common ancestor of living mammals. But “hair” is not one thing. It is a complex of building blocks, including structural genes (like keratin) and developmental processes. Today, scientists can decompose a trait, like hair, into its components and study the evolutionary history of each part separately, tracing the parts through various genomes.

What do we expect for the evolution of hair’s components? Figure 1 suggests that “hair” and all its components arise at the same time, near the origin of mammals. The origin of “hair” on figure 1 can be considered a first-pass hypothesis for the origins of ALL the components of hair. If hair itself originated near the origin of mammals, a logical idea is that the components originated then too.

Today, we can test this first-pass hypothesis because we know some of the molecular components of hair. A particularly important part is “hair keratin”. Mutate this protein and the hair built from that mutant protein is fragile and brittle. The expectation based on figure 1 is that hair keratin proteins originated with hair itself. But the discovery of these genes in an anole indicates an earlier origin for this component. In other words, components of hair originated before hair itself. In this case the protein “hardened” by mutations to cysteine amino acids that may have functioned to molecularly harden the proteins. Since these changes were later useful in the structure of hair, they may be considered exaptations, features that originated for functions other than current utility: keratins may have hardened before that feature became useful for hair formation. [Note for scientific accuracy – the biochemistry of the anole protein has not been studied, so while it is cysteine rich, we don’t know yet if the anole protein is ‘hardened’].

This work also illustrates that in evolution, new things do not appear from nowhere [see my post Coming to Grips]. In evolution, new things come from the duplication/differential modification and recombination of existing parts. Morphologists know this, as one dominant idea about the origin of hair is that hair evolved by modification of scales. Hair keratin is not expressed in anole scales, so the scale hypothesis is not supported by the new PNAS paper. Also unfortunate for the scale hypothesis is the fact that the fossil record retains no transitional forms between scale and hair. Even though morphological relatives of hair are ambiguous, the molecular relatives in this case are clear. Hardened keratin comes as two types, which share an evolutionary relationship, and hardened keratins may share an evolutionary relationship with soft keratins, proteins that are present in numerous tetrapods, and therefore have a more ancient origin than the hard variety. In sum, keratin has an ancient heritage, and through gene duplications and differential modification, two related groups of these proteins have specialized as hair keratins. Fascinatingly, some of the hair keratin modifications pre-dated hair itself.

If you are interested phylogenetic analyses of trait evolution, and the evolutionary history of trait components, this is a common theme of research in my lab.

We’ve found:

Synaptic components are present in sponges and therefore may predate synapses. [paper] [blog]

Phototransduction components were first assembled for vision in the eumetazoan ancestor (cnidaria + bilateria), yet some components pre-date animals [blog] [blog] [paper] [paper]

See also: Red Herring Blog

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