Last month, I started a series on the topic of limb evolution, here at Quintessence of Dust. That series has been transformed (through a series of intermediates) into a series of posts at the BioLogos site. The first installment is now up, and it provides an expanded introduction to the topic and a little historical context. Subsequent posts will tackle fossils, developmental biology, genetics, the explanatory role of design, and related themes.
So go check out the introduction, and feel free to contribute comments, questions and suggestions here. And enjoy the image below, from Wellcome Images, which is featured in the post at BioLogos. Cool, huh?
The discovery of deep homology was a milestone in the history of evolutionary thought. Anatomical structures in distantly related organisms, structures with only the barest of functional similarities, were found to be constructed under the influence of remarkably similar genetic pathways. The original and classic example from 1989 involves genes controlling pattern in both insects and mammals – the famous Hox genes. Another great example emerged from the study of limb development and evolution in vertebrates, work beautifully described by Neil Shubin in Your Inner Fish.
The idea that the limbs of various animals are homologous – meaning that they are variations on a theme inherited from common ancestors – is certainly not new, with roots in the exploration of 'archetypes' by the great Sir Richard Owen. But deep homology goes, well, deeper, suggesting that even basic themes like 'limb' or 'eye' or even just 'thing-sticking-out-of-the-body-wall' can be identified and seen to be conserved throughout the biological world. And, importantly, deep homology points to genetic mechanisms that underlie basic themes, structural concepts so distinct that they would not be judged to be related by structural criteria alone. Consider, for example, limb development in vertebrates.
It is of course a cliche to state that eukaryotic cells (i.e., cells that are not bacteria) are complex. In the case of an animal, tens of thousands of proteins engage in fantastically elaborate interactions that somehow coax a single cell into generating a unique and magnificent organism. These interactions are often portrayed as exquisitely precise, using metaphorical images such as 'lock-and-key' and employing diagrams that resemble subway maps.
Many of these interacting proteins are enzymes that modify other proteins, and many of those enzymes are of a particular type called kinases. Kinases do just one thing: they attach phosphate groups to other molecules. This kind of modification is centrally important in cell biology, and one way to tell is to look at how many kinases there are: the human genome contains about 500 kinase genes.
Now, kinases tend to be pretty picky about who they stick phosphate onto, and this specificity is known to involve the business end of the kinase, called the active site. The active site is (generally) the part of the kinase that physically interacts with the target and transfers the phosphate. You might think that this interaction, between kinase and target, through the active site, would be by far the most important factor in determining the specificity of kinase function. But that's probably not the case.
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