BookBlog

And as for already extinct creatures....

A few weeks back, I wrote about programs that model the development of plants. If you change the parameters of the development algorithm you generate shapes that resemble different types of plants.

Following that thread, I recently read Shapes of Time: The Evolution of Growth and Development. This is a fascinating book that looks at the mechanisms of development in animals, and how those mechanisms affect evolution.

Like the plant models on screen, developing embryos in real life follow a program, where small changes in key parameters generate major changes in shape. There's not one program, but several; during the first phase of growth, parameters are controlled by the egg, later on by the chemical environment in the embryo; still later, by hormones, and by the ratio of cell growth to cell death. In all of these stages, changes in the quantity and timing of key parameters create changes in development.

• In the fruit fly, the "bicoid" gene within the fertilized egg
  controls the creation of a modular body. "A gradient of
  decreasing concentration of the protein from head to tail controls
  the pattern of segmentation (p. 50)." Where the protein is found
  in high concentration, the head develops; where the concentration
  is moderate, the thorax develops; where the concentration is low,
  the abdomen develops.


• Next, the concentration of the bicoid protein activates Hox genes,
  which control body segment development by changes in the position,
  timing, or level of their expression. For example, Hox genes
  control a protein that inhibits limb development; turn up the
  concentration of that protein, and the number of limbs declines
  from many in early arthropods to six in modern insects.


• In a later stage of development, the chemical environment in the
  cell controls development. Increasing the concentration of one
  molecule by 1.5 times causes the molecules to develop into muscle
  cells rather than skin cells.


• Later on, hormones control growth. The pace of growth and timing
  of of life stages affects animal size and behavior. In ants, for
  example, the timing of exposure to hormones controls the emergence
  of different castes of ants. Ants that are exposed to more
  juvenile hormone grow for longer, and become large, fierce
  soldiers instead of smaller, more docile workers.


• Another mechanism is the reduction in the rate of cell death.
  Fingers and toes arise because of the rapid death of cells between
  the digits; a slower rate of cell death results in webbed feet (as
  in ducks and turtles).

Changes in the developmental program may help to explain the emergence of new forms - for example, the evolution of four-legged creatures from fish, according to one recent hypothesis. Fins and limbs arise from the same underlying structure, but the growth parameters are controlled differently. A limb bud has both mesodermal cells (which evolve into flesh and bone), and ectodermal cells, which evolve into skin. In fish, the ectoderm rapidly folds over, halting the growth of mesoderm, and further growth is the skinlike tissue of a fin. In lobe-finned fishes, which represent an intermediate evolutionary step, the mesoderm grows for longer before the ectoderm folds, resulting in a fin that has a stub of flesh and bone, and an extension of fin. In tetrapods, the mesodermal growth continues for much longer, creating a long structure of flesh and bone; with a remnant of nail, claw or hoof at the end.

Changes in the developmental program enable organisms to adapt to new niches. In western Australia, along the sloping bed of the ocean shelf, there can be found fossil brachiopods that become progressively younger-looking as the gradient ascends. The pedicle (sucker-foot) is larger relative to the rest of the body in younger creatures; a slower growth rate would result in adults who were better able to stick to the rocks in wave-wracked shallow waters.

The application of this theory to the evolution of humans is quite fascinating, but this post is quite long enough; read the book if you're interested; or ask me and I'll summarize :-)

There were two main things about the book that were interesting to me.

• First was the concept and the illustration of the algorithm of
  development, from egg to adult organism.
  
• Second is the implication of these algorithms for evolution. It seems
  pretty surprising that small mutations and genetic recombinations can
  generate large change in a relatively short time. It's less strange when
  you think about the development process, where small changes can have
  big effects in the resulting organism; and changes that result in
  competitive advantage are passed on.

That's what I liked about the book. The author's interests were less computational -- the main thesis of the book is about a debate in the field of biology that has been raging since Darwin. The debate is about whether evolutionary development represents "progression" from simplicity to complexity, or "regression" from complexity to simplicity.

Ernst Haeckel, the 19th century biologist who coined the term "biology", theorized that "ontogeny recapitulates phylogeny." According to this theory, development retraces the steps of evolution; embryos of mammals pass through developmental stages that resemble worms, then fishes, then reptiles, then the ultimate mammalian stage. The theory was influenced by an ideology that saw evolution as progression to ever-greater levels of complexity, with humans, of course, at the top of the chain. This theory reigned as scientific orthodoxy until the 1930s.

The problem with the theory is that there is plenty of evidence that contradicts it. In the '30s, biologists Walter Garstang and Gavin de Beer advocated the opposite theory, pedomorphosis. This theory proposed that as organisms develop, they become more like the juveniles of the species. There is plenty of evidence showing this pattern. For example, some species of adult ammonites have shapes that are similar to the juveniles of their ancestor species. According to this theory, human evolution is the story of Peter Pan; we are chimps who never grow up.

Following Stephen Jay Gould, McNamara thinks both sides are right; and he supports Gould's thesis with troves of evidence from many species across the evolutionary tree. Organisms can develop "more" than their ancestors, by growing for a longer period of time, starting growth phases earlier, or growing faster. Or organisms can appear to develop "less" than their ancestors, by growing for a shorter period of time, starting growth phases later, or growing more slowly.

McNamara romps through the animal kindom, from trilobites to ostriches to humans, giving examples of evolution showing that a given species has some attributes that represent extended development, and others that represent retarded development compared to their ancestors. Not being socialized as a biologist, the debate has no charge for this reader. It makes perfect sense that the development program has parameters that can be tuned both up and down!

McNamara's academic specialty is fossil sea urchins, while his day job is a museum of paleontology in Australia. I suspect that the pedagogical impulse of the museum job shows in the book. He's not a populist on the Stephen Jay Gould scale, but the book its decently written (though it could be better edited), and provides enough context so a non-specialist reader can read it quite enjoyably.

I liked it a lot, and plan to follow up with more on related topics, perhaps:

• The Evolution of Complexity by Means of Natural Selection, by John Tyler Bonner


• Acquiring Genomes, a Theory of the Origins of Species, by Lynn Margulis and Doron Sagan, and


• The Self-Made Tapestry: Pattern Formation in Nature, by Philip Ball.

If you're familiar with the topic and have tips for a curious reader, let me know.

Posted by alevin at October 23, 2002 01:37 PM

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