Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom

Developmental biology is the study of how living things unfold from a single fertilized cell to a fully functioning organism. Back in the mid-Pleistocene, when I was a zoology undergrad, developmental biology consisted of embryology and... well, that was pretty much it. In the intervening millennia, science has made major advances in genetics and gene expression, leading to the expansion of a new field: evolutionary developmental biology, or Evo Devo for short. Paleontology, genetics, embryology, molecular biology, and comparative anatomy all rub shoulders in Evo Devo. Finally evolution can be told not in just-so stories, but in step-by-step genetic transcriptions and protein translations. Rather than speculating on how insects developed flight, we can see exactly which genes are involved in making wings, and look at what those same genes do in flightless insects.

Genes are often first identified when something goes wrong; when a mutation causes a developing organism to be malformed. Thus, many genes are named not for their normal function, but what happens when they mutate. The eyeless gene is actually the gene that initiates eye formation in most animals, but it was named when it was found that mutations resulted in - you guessed it - eyeless fruit flies. Many strangely named genetic characters populate Endless Forms Most Beautiful, including tinman which is involved in making hearts (get it? -- think Dorothy and Toto) and Sonic hedgehog, which is involved in limb production of many animals, including us.

One of the big revelations from Evo Devo is genomes contain toolboxes that are largely conserved throughout evolution. A small group of genes control much of an animal's embryological development, and these genes are similar from fruit flies to humans. The genes that control development of our limbs are the same family of genes that control insect limb development. These toolbox genes are also usually arranged on chromosomes in the order of the body regions they control. They are called homeobox genes, or Hox genes for short.

New and modified structures often come not from new genes, but from new uses of old genetic tools. The genes that make insect wings started out making gill branches in aquatic arthropods. To understand how this could work, first a bit of anatomy. Animal bodies develop in segments. In many invertebrates these segments are easy to see, even externally. Each pair of a millipede's legs comes from a segment, as a famous example. All animals develop this way, even us. Genes can perform different tasks depending on which type of segment they are in; a head segment, a body segment, a limb segment, etc. The difference between limbs from species to species is as much due to different switches as different genes. Switches are another area of great advancement; back when I took genetics they knew about repressor proteins and activator proteins, but just a few examples were known. Now it appears that activation and repressor switches play a large role in development, and a single gene can trigger cascades of switches. The homeobox genes largely consist of sequences which encode proteins that bind to other parts of the genome, causing those other genes to either be expressed or not be expressed.

Switches also disproved an idea of Alan Turing's, one of the rare occasions he was wrong about something. In the early 1950s Turing wrote a fascinating paper suggesting animal stripes could be obtained via different chemicals diffusing through a developing embryo. Every book on chaos mentions it as an example of how a simple method can yield complex patterns. But it turns out that switches are how nature usually makes stripes. You start with a gene that puts a dark pigment across the organism, and then switch that gene off in alternate segments of the body. Voila, stripes. This theme of starting with plenty and then genetically switching things off is a common theme in Evo Devo. Paleontologist Samuel Williston framed this as a law, stating, "it is [also] a law in evolution that parts in an organism tend toward reduction in number, with the fewer parts greatly specialized in function." Carroll shows how in specialized organisms the genes for making many parts (teeth, limbs, etc.) are often still there in specialized animals, but they are just switched off during development.

I feel bad that I'm not doing this book (or subject) justice. It really is interesting stuff, and this is coming from someone who found embryology dull dull dull. It takes more than a few paragraphs to explain it well, though, and Carroll is a much better explainer than I. The title comes from Darwin's Origin of Species: "...from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved." Want to learn about butterfly wings and their patterns? It's in here. Ever wondered why we don't look more like chimpanzees if we share over 98% of our DNA? That's in here too. Want to know how spider spinnerets, spider book lungs, and insect wings all evolved from genes that originally made gill branches? Yup, that's here. Want to see a wicked picture of a monster Cyclops lamb? Check. I know a book on developmental biology isn't the sort of thing that usually leaps off the shelf into people's hands, but if you are at all interested in how our genetic toolboxes work, Endless Forms Most Beautiful is well worth a reading or two.

• Read more about this book