Browser-final 10/12/05 9:50 AM Page 22 formations, with insightful illustrations quirky and rambling, but somehow al- and particle/wave dualities. To get ready for provided for assistance. Those who stick ways intriguing. that day, aspiring theoretical physicists are with it are rewarded with a realistic peek How seriously should we take all this encouraged to buy a copy, for there’s much at physicists at play, seeing how they can talk of vibrating strings and parallel uni- inspiration to be found here. absorb a new concept and fashion poten- verses? Hypotheses in high-energy physics tial solutions to long-held mysteries. rise and fall on the Internet these days, Journalist and author Marcia Bartusiak is a visit- Warped Passages is more challenging than sometimes in a matter of hours. But I can ing professor in the Graduate Program in Science most popular science works, but highly imagine getting comfortable with branes Writing at the Massachusetts Institute of Technol- accessible to the physics-minded because and higher dimensions, as some of us are al- ogy. Her latest books are Einstein’s Unfinished of Randall’s lucid explanations. It’s ready accustomed to black holes, relativity, Symphony and Archives of the Universe. Better Living through Evolution The science of novelty and complexity in life forms by daniel l. hartl rançois jacob, one of the pio- are the sequences of neers in molecular biology, was in- such macromolecules terviewed a few years ago for an ed- similar, they are more ucational video now used in similar among more Harvard’s new undergraduate bi- closely related organ- Fology curriculum (Life Sciences 1b: “Ge- isms. In particular, netics, Genomics, and Evolution”). Re- when the sequences flecting on an earlier time, Jacob of macromolecules reminisced: are compared among When I started in biology in the groups of organisms, 1950s, the idea was that the mole- the evolutionary rela- cules from one organism were very tionships among the di≠erent from the molecules from organisms are almost another organism. For instance, always in close agree- cows had cow molecules and goats ment with those in- had goat molecules and snakes had ferred from anatomy, snake molecules, and it was be- physiology, and devel- cause they were made of cow mole- opment. The molecular cules that a cow was a cow. And evidence shows incon- then it turned out, a little bit later, trovertibly that species that certain molecules were very have come into being similar from one species to another, gradually throughout for instance, hemoglobin. The he- the history of life on moglobin of horse or cow or human Earth. Darwin could turned out to be very similar, and not have dreamed of progressively…it turned out that such a spectacular more and more molecules of vari- confirmation of his ous organisms were very similar. theory of descent with The discovery that the subunits of DNA modification. For cre- and proteins have similar sequences in ationists, this must be very di≠erent organisms is one of the least a nightmare, for any appreciated but most important discover- sensible model of cre- ies in twentieth-century biology. Not only ationism would pre- dict cows to have cow Marc W. Kirschner and John C. Gerhart, molecules, goats to have goat molecules, sign,” which asserts that the complexity of The Plausibility of Life: Resolving Darwin’s and snakes to have snake molecules. features such as the vertebrate eye or the Dilemma (Yale University Press, $30). So creationists have shifted ground to molecular motor that drives the flagellum in promote a theory called “intelligent de- bacteria must have arisen instantaneously 22 November - December 2005 Illustration by Dan Page Browser-final 10/12/05 9:50 AM Page 24 as a result of purposeful and intelligent de- ing how environmental and genetic varia- American alligator develop as females if sign. Although the designer is not specified, tion is expressed. incubated at 86°F, but as males if incu- this sly dissimulation is a transparent at- Most evolutionary change actually bated at 91°F. What tips the balance is the tempt to dodge the Supreme Court rulings takes place within a background of quantity of the hormone estradiol that is in Epperson v. Arkansas (1968) and Edwards v. nonevolution. Innovations are actually produced, which acts as a switch that re- Aguillard (1987) holding that the First quite rare. Darwin was well aware of the leases the female developmental program Amendment prohibits any state from re- conservatism and asked already present. quiring teachers to promulgate “the princi- Why, on the theory of Creation, The third feature of organisms that ples or prohibitions of any religious sect or should there be so much variation promotes facilitated variation is ex- dogma” (Epperson v. Arkansas 393 U.S. 97). and so little real novelty? Why ploratory behavior and the stabilization In The Plausibility of Life, Marc W. Kirsch- should all the parts and organs of of essentially random processes. For ex- ner and John C. Gerhart ’58 propose a the- many independent beings, each ample, when neurons grow out from the ory for the origin of variation that de- supposed to have been separately developing spinal cord, they extend their mystifies the evolution of novelty and created for its proper place in na- long, thin axons in essentially random di- complexity. It is a milestone book full of ture, be so commonly linked to- rections. Most of these cells undergo pro- new and insightful ideas. I recommend it gether by graduated steps? Why grammed cell death. The neurons that without hesitation. Most evolution books should not Nature take a sudden survive are those with axons that happen written for the general public are dumbed leap from structure to structure? to have entered the appropriate target tis- down and have little new to say, but It would comfort Darwin to know that sues, where they receive a protein survival Kirschner and Gerhart walk the tightrope evolution at the molecular level is even factor that inhibits the programmed cell with spectacular success, using clear, more conservative than at the morpholog- death. The comings and goings of pro- nontechnical language, brilliantly chosen ical level. The fundamental life processes grammed cell death are important in examples, and numerous illustrations. evolved early and have remained essen- many examples of evolutionary innova- Exploring the evolutionary implica- tially unchanged for the past two billion tion. The embryos of essentially all verte- tions of the developmental mechanisms years at least. These conserved core brates, for example, have digits connected that have been discovered in the past 30 processes include the major metabolic by webs of skin. In most mammals and years, Kirschner (Walter professor of sys- pathways for synthesizing or degrading most birds, cells in the interdigital re- tems biology and head of that department small molecules, the structural and regu- gions of the feet usually undergo pro- at Harvard Medical School; see “Seeing Bi- latory components of the cell, the mecha- grammed cell death, but in the develop- ological Systems Whole,” March-April, nisms of DNA replication, transcription ment of ducks and geese, a developmental page 67) and Gerhart (professor of the of DNA into RNA, use of RNA to specify signal cancels the death program, result- graduate school, division of cell and devel- the structure of proteins (the genetic ing in the seemingly marvelous webbed opmental biology, at the University of Cal- code), and the regulatory mechanisms feet so adapted to an aquatic life style. ifornia, Berkeley) conclude that the ori- that control transcription and transla- Evolutionary novelty is also facilitated gins of novelty and complexity are tion. Among multicellular organisms, by the compartmentation of the embryo. implicit in the physiology and develop- most of the novelties in body plan had al- Animal embryos at a middle stage of de- ment of organisms themselves. The key is ready occurred by the Cambrian period velopment form a few dozen compart- what they call “facilitated variation.” By (543–490 million years ago), although ments with relatively weak linkages this they mean that an organism does not some innovations, such as paired ap- among them. As development proceeds, merely tolerate environmental perturba- pendages in vertebrates, appeared in these are subdivided successively into tions or developmental accidents, but in more recent periods, the Ordovician smaller compartments nested within fact adjusts to the disturbances and incor- (490–443 million years ago) or Silurian larger ones. In vertebrate development, porates them into its physiology or devel- (443–417 million years ago). there may be as many as 200 kinds of opment. This bu≠ering facilitates varia- While the core life processes are con- compartments. Any of these can grow or tion in traits by channeling environmental served, a riot of variation and innovation shrink in size, or change in shape, quite or genetic irregularities into integrated is found in morphology, development, independently of the others, which al- pathways of response. Furthermore, ran- and behavior. Kirschner and Gerhart at- lows developmental and evolutionary dom inputs in the form of environmental tribute this to several general features of change while maintaining overall coordi- perturbations or genetic mutations do not physiology and development. First, ge- nation consistent with the body plan of produce random outputs, because the netic regulatory signals are simple and the organism. Examples abound, such as outputs are shaped by the organism’s weak (often consisting of extremely short the neck of the gira≠e, the trunk of the adaptive responses. Although genetic mu- stretches of DNA that regulatory proteins elephant, or the antlers of the deer. Most tations may be random in their e≠ects on can bind with), and therefore can readily important: the same genes, signaling mol- the DNA sequence of an organism, facili- arise or disappear through mutation.