I had the most random epiphany over my morning tea today. I don’t even know what got me thinking about the Cambrian explosion (as if I needed a reason…). Might have been remembering something from the Euro Evo Devo conference I recently went to. (I kind of wanted to post about that, because I saw some awesome things, but too much effort. My brain isn’t very cooperative these days.)
I was thinking about explanations of the Cambrian explosion and remembering how the relevant chapter in The Book of Life (otherwise known as the book that made me an evolutionary biologist) tried to make it all about Hox genes. It’s an incredibly simplistic idea, and almost certainly wrong given what we now know about the history of Hox genes (and animals)*. At the time, and for a long time afterwards, I really wanted it to be true because it appeals to my particular biases. But I digress.
Then it dawned on me just how new and shiny Hox genes were when this book was written. I thought, holy shit, TBoL is old. And how far evo-devo as a field has come since!
The Book of Life was first published in 1993. That is less than a decade after the discovery of the homeobox in fruit fly genes that controlled the identity of segments (McGinnis et al., 1984; Scott and Weiner, 1984), and the finding that homeoboxes were shared by very distantly related animals (Carrasco et al., 1984). It was only four years after the recognition that fly and vertebrate Hox genes are activated in the same order along the body axis (Graham et al., 1989; Duboule and Dollé, 1989).
This was a HUGE discovery. Nowadays, we’re used to the idea that many if not most of the genes and gene networks animals use to direct embryonic development are very ancient, but before the discovery of Hox genes and their clusters and their neatly ordered expression patterns, this was not at all obvious. What were the implications of these amazing, deep connections for the evolution of animal form? It’s not surprising that Hox genes would be co-opted to explain animal evolution’s greatest mysteries.
It also occurred to me that 1993 is the year of the zootype paper (Slack et al., 1993). Slack et al. reads like a first peek into a brave new world with limitless possibilities. They first note the similarity of Hox gene expression throughout much of the animal kingdom, then propose that this expression pattern (their “zootype”) should be the definition of an animal. After that, they speculate that just as the pattern of Hox genes could define animals, the patterns of genes controlled by Hoxes could define subgroups within animals. Imagine, they say, if we could solve all those tough questions in animal phylogeny by looking at gene expression.
As always, things turned out More Complicated, what with broken and lost Hox clusters and all the other weird shit developmental “master” genes get up to… but it was nice to look back at the bright and simple childhood of my field.
(And my bright and simple childhood. I read The Book of Life in 1998 or 1999, not entirely sure, and in between Backstreet Boys fandom, exchanging several bookfuls of letters with my BFF and making heart-shaped eyes at long-haired guitar-playing teenage boys, I somehow found true, eternal, nerdy love. *nostalgic sigh*)
*Caveat: it’s been years since I last re-read the book, and my copy is currently about 2500 km from me, so the discussion of the Cambrian explosion might be more nuanced than I remember. Also, my copy is the second edition, so I’m only assuming that the Hox gene thing is there in the original.
Carrasco AE et al. (1984) Cloning of an X. laevis gene expressed during early embryogenesis coding for a peptide region homologous to Drosophila homeotic genes. Cell 37:409-414
Duboule D & Dollé P (1989) The structural and functional organization of the murine HOX gene family resembles that of Drosophila homeotic genes. The EMBO Journal 8:1497-1505
Graham A et al. (1989) The murine and Drosophila homeobox gene complexes have common features of organization and expression. Cell 57:367-378
McGinnis W et al. (1984) A conserved DNA sequence in homoeotic genes of the Drosophila Antennapedia and bithorax complexes. Nature 308:428-433
Scott MP & Weiner AJ (1984) Structural relationships among genes that control development: sequence homology between the Antennapedia, Ultrabithorax, and fushi tarazu loci of Drosophila. PNAS 81:4115-4119
Slack JMW et al. (1993) The zootype and the phylotypic stage. Nature 361:490-492