It indicates that many features of bones and teeth arise as by-products of other traits during an individual's skeletal development. Wood, who until recently was an avid and influential evolutionary tree builder, now doubts the accuracy of cladistics as currently practiced. Such debates will prove fruitless, not to mention endless, unless scientists manage to discern how bones develop and which of their features reflect direct genetic effects, White says. We need to find out how these traits develop and which ones truly have an evolutionary signal. Lovejoy's group devotes particular attention to vertebrate DNA sequences known as homeobox genes, which biologists have studied over the past 15 years.
Homeobox genes control the general shape of the fetal body.
These genes also trigger a series of biological processes that foster the growth of integrated sections of the skeleton rather than the development of individual bones, Lovejoy asserts. For instance, the same two homeobox genes appear to regulate the growth of both the vertebrate forearm and all fingers except the thumb, whereas a couple other of these genes participate only in thumb growth, Lovejoy and his colleagues report in an upcoming Evolution and Development . Relatively subtle alterations of these genes or their protein products at various times could easily have accounted for the evolution of hand proportions typical of modern primates and of our fossil ancestors, the researchers theorize. Lovejoy and his colleagues used chemical probes injected into mouse embryos to establish that one pair of homeobox genes displays a distinctive molecular footprint in the thumb bones, while another pair leaves a unique calling card in bones of the forearm and the remaining four fingers. In a sample of 65 skeletons from people, apes, and monkeys, Lovejoy's team then found that the adult forearm is longer or shorter in concert with corresponding size differences in all fingers except the thumb.