Since its humble beginnings as a single cell, life has evolved into a spectacular array of shapes and sizes, from tiny fleas to towering Tyrannosaurus rex, from slow-soaring vultures to fast-swimming swordfish, and from modest ferns to alluring orchids.
But just how such diversity of form could arise out of evolution's mess of random genetic mutations has remained one of the most fascinating questions in evolutionary biology.
Now finally, after more than a century of puzzling, scientists are finding answers from a surprising quarter, evo-devo. Just coming into its own as a science, evo-devo is the combined study of evolution and development, the process by which a nubbin of a fertilized egg transforms into a full-fledged adult. And what these scientists are finding is that development, appears to have been one of the major forces shaping the history of life on earth.
Evo-devo researchers are finding that the evolution of complex new forms, rather than requiring many new mutations or many new genes as had long been thought, can instead be accomplished by a much simpler process requiring no more than tweaks to already existing genes and developmental plans.
Master genes
Stranger still, researchers are finding that the genes that can be tweaked to create new shapes and body parts are surprisingly few. The same DNA sequences are turning out to be the spark inciting one evolutionary flowering after another. "Do these discoveries blow people's minds? Yes," said Dr. Sean B. Carroll, biologist at the Howard Hughes Medical Institute at the University of Wisconsin, Madison.
The development of an organism is controlled by a hierarchy of genes, with master genes at the top controlling a next tier of genes, controlling a next and so on. But the real interest for evolutionary biologists is that these hierarchies not only favor the evolution of certain forms but also disallow the growth of others, determining what can and cannot arise not only in the course of the growth of an embryo, but also over the history of life itself.
"It's been said that classical evolutionary theory looks at survival of the fittest," said Dr. Scott F. Gilbert, a developmental biologist at Swarthmore College, adding, "evo-devo looks at the arrival of the fittest."
Charles Darwin saw it first. He pointed out well over a century ago that developing forms of life would be central to the study of evolution. Little came of it initially. But the advent of molecular biology reinvigorated the study of development in the 1980s, and evo-devo quickly got scientists' attention when early breakthroughs revealed that the same master genes were laying out fundamental body plans and parts across the animal kingdom
In one of the most exciting of the new studies, a team of scientists led by Dr. Cliff Tabin, a developmental biologist at Harvard Medical School, investigated a classic example of evolution by natural selection, the evolution

Same hurdles, same solutions! Evo-devo has also begun to shine a light on a phenomenon with which evolutionary biologists have long been familiar, the way in which different species will come up with sometimes jaw-droppingly similar solutions when confronted with the same challenges. Among the placental mammals of the Americas and the marsupials of Australia, for example, have evolved the same sorts of animals independently: beasts that burrowed, loping critters that grazed, creatures that had long snouts for eating ants, and versions of wolf. In the same way, the cichlids have evolved pairs of matching species, arising independently in separate lakes in Africa. In Lake Malawi, for example, there is a long and flat-headed species with a deep underbite that looks remarkably like an unrelated species that lives a similar lifestyle in Lake Tanganyika. There is another cichlid with a bulging brow and frowning lips in Lake Malawi with, again, an unrelated but otherwise extremely similar-looking cichlid in Lake Tanganyika. The same jaws, heads, and ways of living can be seen to evolve again and again.

of Darwin's finches on the Galapagos Islands. Darwin did not realize for some time that these birds were all finches. He should be forgiven, however. For while the species are descendants of an original pioneering finch, they no longer bear its characteristic short, slender beak, which is excellent for hulling tiny seeds.
Adapting to the strange new foods of the islands, some finches have evolved taller, broader, more powerful nut-cracking beaks; the most impressive of the big-beaked finches is Geospiza magnirostris. Other finches have evolved longer bills that are ideal for drilling holes into cactus fruits to get at the seeds; Geospiza conirostris is one species with a particularly elongated beak.
Scientists had assumed that the dramatic alterations in beak shape, height, width and strength would require the accumulation of many chance mutations in many different genes. But evo-devo has revealed that getting a fancy new beak can be simpler than anyone had imagined.
What Tabin and colleagues found, when looking at the range of beak shapes and sizes across different finch species, was that the thicker and taller and more robust a beak, the more strongly it expressed a gene known as BMP4 early in development. The BMP4 gene produces the BMP4 protein, whichsignal cells to begin producing bone.
But BMP4 is multitalented and can also act to direct early development. To verify that the BMP4 gene itself could indeed trigger the growth of grander, bigger, nut-crushing beaks, researchers artificially cranked up the production of BMP4 in the developing beaks of chicken embryos. The chicks began growing wider, taller, more robust beaks similar to those of a nut-cracking finch.
In the finches with long, probing beaks, researchers found at work a different gene, known as calmodulin. The more that calmodulin was expressed, the longer the beak became. When scientists artificially increased calmodulin in chicken embryos, the chicks began growing extended beaks, just like a cactus driller.
NYT News Service