It's old news by now that human beings' closest relative in the animalkingdom is the chimpanzee. This primate, (Pan troglodytes), sharesmore DNA with Homo sapiens than - as previously thought - itdoes with the gorilla (Gorilla gorilla).It's also been thought that birds are more closely related to mammalsthan to reptiles. This particular phylogenetic belief was reversedyesterday by the scientist who in recent years asserted the bird-mammal connection by DNA sequence analysis.He is molecular phylogeneticist S. Blair Hedges of Pennsylvania StateUniversity, author of a paper in the Proceedings of the NationalAcademy of Sciences (PNAS), March 29, 1994. In it he states: "DNAsequences from four slow-evolving genes . . . now provide strongstatistical support for a bird-crocodilian [rather than a bird-mammalian]relationship."Hedges attributes his switch in re-assigning birds from the mammalianto the reptilian branch on the tree of life as thanks to more extensiveDNA sequence homology comparisons, and stronger statisticalresolution.Before DNA probes came on the scene a decade or so ago,paleontologists had only fossil bones and teeth for comparing modernand ancient forms of life, Hedges explained. DNA analysis itself hasgone through three stages of informativeness, from restriction fragmentlength polymorphisms (RFLPs), to nucleic acid hybridization to genesequencing."Clearly," Hedges told BioWorld, "not enough sequence data has beenavailable up to now to answer the question: Are birds closer tomammals or reptiles?" To close this gap, he "set out the last couple ofyears to collect another three kilobases of sequence data." He chosespecific mitochondrial genes that evolve very slowly, so theinformation would be relevant to his problem.Libraries of these are at hand for a broad spectrum of mammals andbirds, but to scope the reptilean world he sequenced genes from thecommon crocodilian (Alligator mississipiensis), from a lizard, a rareliving-fossil reptile, the tuatara (Sphenodon punctatus) of NewZealand, and a turtle. All of these descend from ancestors that lived350 million years ago.Hedges sequenced long stretches of all these genes manually, andamplified his DNA directly from PCR, not by cloning.When he ran his molecular data through the "bootstrap" statisticalprogram, "What I came up with was the birds clustering very strongly- with 97 percent confidence - with crocodilians, not mammals."He continued, "This origin of birds is interesting, because birds havebeen claimed as living representatives of dinosaurs, now extinct. Thenext most closely related group are the crocodilians. So for peopleworking on dinosaur DNA, including myself, this allows us to focus ongenetic sequences, the closest relatives of dinosaurs being birds."In the Jurassic Age of Dinosaurs 150 million years ago, he noted,archaeopteryx, precursor to the birds of today, left fossils coeval withTyrannosaurus rex. Like "Jurassic Park" deja vu, Hedges is now, infact "looking at ancient DNA from dinosaur bones and biting flies inamber."One compelling reason that he cites for the widely held opinion thatbirds related most closely to mammals is the fact that chickens andhumans both have four-chambered hearts. Given his new, even morecompelling DNA support for the bird-reptile link, he said, "we mustnow conclude that the four-chambered heart evolved independently, byconvergence."The once-held bird-mammal relatedness, he quipped, "is now strictlyfor the birds."Hedges measures phylogenetic values by the fairly new "bootstrap"statistical system, which estimates variance "by repeatedly sampling,with replacement, your data set in random fashion. That is, you samplethe real data, but choose the sites randomly. When you do this one ortwo thousand times, you get your mean, and thence your variance."Applied to his problem, Hedges said, "When I put all the DNAsequence data together, it worked out at 5,250 sites, with 100 percentsupport for birds and crocodilians, less that 1 percent for mammals.The Academy member who communicated Hedges' paper to PNAS isCharles Sibley, honorary curator of ornithology at Yale University'sPeabody Museum. It was he, a decade ago, who proved by DNAhybridization that chimps relate more to humans than to gorillas.Sibley told BioWorld, "I believe the DNA stuff more than I believemuscles or bones."Hedges' latest finding, Sibley said, "represents traditional sequencing.He's got long 16S and 18S ribosomal RNA, so the basis for thisconclusion is more solid than his previous one - being excruciatinglycareful to cover all the bases; no pun intended." Sibley added, "Afterall, birds are feathered reptiles."Beyond feeding Homo sapiens' innate curiosity to find their roots,what practical purpose is served by studies of bird/reptile orbird/mammal relatedness?Sibley's first answer: "Knowledge for its own sake is one of thegrandest things that humans do."He also thinks it will find practical applications "in the realm ofbiodiversity, so that people who are concerned about the destruction ofthe planet, and the rapid disappearance of many species, includingbirds, have additional information. Because, if you can't save aparticular species, the next best thing is to try to save its closestrelative."He summed up: "This business of trying to figure out who's related towhom, and by how much, among the organisms on this planet is notlacking in practicality, but as far as turning it into dollars for some drugcompany, I don't think we can do that."
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.