The basic genetic software that drives the development of organisms is very old, and is shared in common across the animal kingdom. The software is modular, with components that govern the development of eyes, limbs, and hearts. The genetic program that builds a multi-faceted fly eye is mostly the same as the program that builds a human eye.
Components are re-used to build different body parts — the module that makes fingers and toes is re-used to make the spots on a butterfly. The genes are the same; the component architecture is the same, and the detail of the program itself is different.
This is beautiful science — general laws holding together a vast number of seemingly unrelated facts. And it’s new science. Until recently biologists theorized that eyes and limbs had developed independently in different families of organisms. The basic discoveries were made about 20 years ago, and much of the detail has been added in the last decade.
Analysis of the newly sequenced genome in fruit flies, frogs, mice, and humans revealed that organism were more similar than expected — humans and mice share 97.5% of their genes. Scientists studying developing embryos were able to identify the genes that launch developmental programs, and to discover the similarities between developmental programs in different types of organisms. With a picture of the developmental program in live organisms, paleontologies have been able to look at old forms and build theories of how changes in program drove changes in animal form.
“Endless Forms” is a nicely-written, layman-friendly survey of evolutionary development — a new synthesis of genetics, embryology, and paleontology — by one of the field’s pioneers. The primary strength of the book is that it summarizes the last 20 years of scientific discoveries, and provides an overview of the core body-building genetic patterns.
The survey is complementary to two more specific books that I’ve read on the related subjects in the last few years.
Shapes of Time, written by a Kenneth McNamara, a paleontologist in Western Australia, goes into more detail about the the algorithms used in the development of embryos and young organisms. And “Shapes” has an ambitious thesis about how the escalation and de-escalation of the developmental timeclock has guided the creation of new species.
The Symbolic Species, by Terence Deacon, a brain scientist at Harvard Medical school, describes the process by which nerves grow in the developing primate brain.
The primary argument of Deacon’s book, about how humans developed symbolic thought, is speculative to the point of fanciful, and is not well-constrained by his science. The science itself is interesting enough. Decond compares the wiring of human brains with the brains of chips and birds. Deacon argues that the distinctive difference between humans and other species with communication skills isn’t the size of the brain, or simply the addition of a new, larger, forebrain component, but the rerouting of the nerve wiring from the centers of emotion and motor control, through the newer regions of logic and planning.
The books by Deacon and McNamara are more uneven in editorial quality, but more intellectually savory. I get a real kick out of the exploration of the algorithm — the detail of how development happens — and the logical analysis of evolutionary mechanism –the hypothesis about how variations in the algorithm of development affects the form and behavior of the organism.
Endless Forms is a really good way to get a big picture overview of evo devo science, and an entertaining fast-forward through the plot of scientific discovery, without requiring the level of commitment to get through one of the graduate-level survey textbooks in the field.
Carroll ends with a pitch for using evolutionary development as part of the basic teaching of evolution in schools. The history and mechanism of the evolution of form is dramatic and intellectually persuasive. It’s ironic that evolution is becoming more politically controversial, even as science makes such progress at understanding how evolution happens.