Arthur: Creatures of Accident

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Wallace Arthur, Creatures of Accident : The Rise of the Animal Kingdom. New York, Hill and Wang, 2006. x + 255 pages, with glossary, "further reading", and index.

The title of this book alludes to a question that the author feels is rarely addressed: why are plants and animals so complex? Nature has evolved an amazing and vast array of remarkably complex organisms to populate the Earth, going far beyond the already complex single-cell creatures. And, there is that question that so many still find perplexing about evolution through natural selection: how could it come to produce such complexity as we see in the natural world?

We may not have all the answers yet, but another level of understanding has arrived through recent research and understanding in evolutionary developmental biology, sometimes "evo-devo" for short. Where the focus was once on how adult animals were similar or different, this new understanding involves development: how a bunch of undifferentiated cells can grow into a complex organism made of many different types of cells. It's come to seem obvious remarkably quickly that evolution does not operate on adult animals; evolution effects its changes in the development of animals.

A corollary of the precise duplication of the genetic material with each cell division is that all cells have a full complement of genes. Since it is the genes that make the main players in the chemistry of a cell, namely- proteins—both enzymes and others—there is a puzzle here. If all cells have the same genes, how do they become so very different from each other? How, for example, do some cells become muscle cells and others nerve cells? In other words, how do cell differentiate, to use the technical but self-explanatory term?

The answer to this question is that although almost all cells have a full complement of genes, different genes are switched on (and thus making proteins) in different cases. For example, the genes that make the contractile proteins that belong in muscle cells are switched on in those cells but switched off in others, such as brain cells. At an early stage in our understanding of these things, this was known as the variable gene activity theory of cell differentiation. Like the cell theory, the variable gene activity theory has stood the test of time, and is known to be generally true (say, again, in more than 99 percent of cases), but with the occasional exception that living systems always seem determined to provide. [pp. 61—62]
When I was a student, some thirty years ago, it was often said that development—that magnificent journey from egg to adult, which most creature embark upon, even though only a minority reach their destination—was the biggest unsolved problem in the whole of biology. And in my view, that was indeed true at the time. In terms of pure, as opposed to applied, biology, there were the cell theory, the theory of natural selection, and Mendel's laws of inheritance, which had recently been given a molecular dimension by Watson and Crick. So there were grand general theories in the areas of what organisms are made of, how they pass on their genes to the next generation, and how, in the longer term, evolution could change both organisms and their genes.

Contrast these remarkable achievements with the state of developmental biology in the same period—the early 1970s. There was no general theory of development. Some biologists were even arguing that there could never be such a theory, because development was a plethora of different subprocesses, making it too bitty to be explained by any grand, sweeping generalizations.

At that time, developmental biology, then more often called embryology, had three main strands—descriptive, comparative, and experimental (respectively: describing development in a particular creature; comparing the development of different creatures; and deliberately perturbing development to see what happens). [...]

As we can see now with the benefit of hindsight, these three strands were not enough. There was a fourth waiting in the wings, without which a general theory of development could not be formulated. This genetic strand did not come into its own until 1984, with the discovery of a gene sequence called the homeobox that forms the core of Chapter 13. [pp. 67—68]

The discovery of the homeobox was a turning point in developmental biology. Before then we understood that genes somehow controlled the appearance of many adult characteristics, and that they did it by specifying proteins. But how was not clearly realized until the homeobox was discovered. It led the way to realizing that DNA, in addition to containing the genes, also contains a number of "switches" that turn gene-expression on and off at very specific times during development (i.e., during the growth of an embryo). The homeobox explained not only how genes are expressed, it indicated how mutation could effect the amazing rise of complexity that we see in animal-kind.

In the early days of molecular genetics, it was discovered that many genes have the short sequence TATA just before their start. To highlight this common sequence in the otherwise different streams of characters, it became accepted practice to draw a box, or rectangle, around it. This practice is helpful, because long strings of characters tend to befuddle the eye—they bury it in too much information and prevent important recurring patterns from leaping out. And so we had the TATA box. Since then, many other commonly recurring sequences of varying lengths have been found, and been called boxes, too. The way they are named varies. Sometimes the name derives from the repeated sequence—as in TATA—but sometimes the box is named after its discoverer. A third approach to naming a box is to base the name on something the repeated sequence does, when it is eventually translated into its protein product. This is the rationale behind the name of my "best box of all," the homeobox. [p. 153]
And so to the homeobox. In the early 1980s, two groups of biologists, one working in Indiana, the other in Switzerland, made an important discovery. Actually, that is putting it mildly: if I had to choose the most important biological discovery of the last half century, this would be it. They discovered that a certain DNA sequence kept showing up in a whole lot of genes in many different animals. This box was much longer than the TATA box. in fact, it consists of abut 180 characters of the DNA alphabet. And although the sequences vary a little, especially across big evolutionary distances, such as that separating mice and flies, the degree of similarity of the sequences is staggering—often more than 90 percent; sometimes more like 99 percent. [p. 154]

This is the heart of the story, of course, the key to developmental biology and the discovery that merged embryology, genetics, and evolutionary theory into "evolutionary developmental biology". This is the second book I've read on evo-devo so comparison seems inevitable. Previously I had read Sean Carroll's Endless Forms Most Beautiful. Neither book suffers for it, though, because they have very different goals.

Arthur's Creatures of Accident elaborates on a select few major concepts without Carroll's wealth of detail that would intimidate some readers and energize others. Arthur carefully avoids a number of details unnecessary to his main themes, because he wants to make a thorough understanding of these core ideas accessible to the broadest audience; Carroll wants to display all the wonders of recent research in evo-devo. Carroll's presentation made me think of an animated speaker in a large lecture hall. Arthur's, on the other hand, was more an intimate conversation between friends in comfortable chairs in a cozy room.

I admit to losing my patience with Arthur's pace at the beginning of the book, but it just took me a bit longer than usual to catch his pace. Fortunately I caught on. Creatures of Accident is not a breathless whirlwind tour. It's more of a friendly, unhurried woodland stroll with a knowledgeable friend who can point out the tiny flowers one would otherwise overlook.

Creatures of Accident is closer to an extended personal essay than a textbook. Behind Arthur's writing is a wealth of experience and understanding that comes through in comfortable prose that is nevertheless scientifically concise and accurate.

By the way, be sure to read through the glossary at the end of the book.

-- Notes by JNS

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