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Title    : Large Gene Study Questions Cambrian Explosion
Author : Barbara K. Kennedy (PIO)
Date    : January 19, 1999


The ancestors of major groups of animal species began populatingEarth more than 600 million years earlier than indicated by theirfossil remains, according to the largest study on the subject usinggene sequences, recently completed at Penn State. The researchsuggests that animals have been evolving steadily into differentspecies for at least 1200 million years, which challenges a populartheory known as the Cambrian Explosion that proposes the suddenappearance of most major animal groups, known as phyla, 530 millionyears ago.

A paper describing the research will be published in the January 22,1999, issue of the Proceedings of the Royal Society of London (SeriesB) by Penn State Undergraduate Student Daniel Y.-C. Wang,Postdoctoral Fellow Sudhir Kumar, and Associate Professor of BiologyS. Blair Hedges.

To gauge the pace of evolution, the research team tested hundreds ofgene sequences to find those that developed mutations at a constantrate over time. "Because mutations start occurring at regularintervals in these genes as soon as a new species evolves--like theticking of a clock--we can use them to trace the evolutionary historyof a species back to its actual time of origin," Hedges explains.

By comparing individual genes in pairs of species, the researchersidentified 75 nuclear genes that had accumulated mutations at afairly constant rate relative to one another during their evolution. The genes were from species representing three major taxonomicgroups, or phyla, of animals (arthropods, chordates, and nematodes),plus plants and fungi.

The scientists then calibrated these molecular clocks to anevolutionary event well established by fossil studies--the divergenceof birds and mammals about 310 million years ago. "A clock isn't anygood unless it is calibrated to a time that everyone else agrees on,"Hedges explains, "and just about everyone agrees on the date whenreptilian ancestors of birds and mammals appeared because it is basedon well-accepted studies of fossils." Using this date as a securecalibration point--and the mutation rate for each of theconstant-rate genes as a timing device--the researchers were able todetermine how long ago each of the species originated.

"Not only are all these genes telling us that a wealth of animalspecies in at least three phyla were already on Earth millions ofyears before their fossils start appearing," Hedges says, "but theyalso are telling us when three of the major kingdoms of livingthings--animals, plants, and fungi-- first diverged from a commonancestor and began evolving down separate evolutionary paths." Thatdate--about 1.6 billion years ago--is the earliest yet obtained bygene studies for this evolutionary event, according to Hedges.

The Penn State team used more than twice as many genes to date theorigin of the three major animal phyla as had been used in any otherstudy since gene sequences first became available in the Genbankpublic databases maintained by the National Institutes of Health(NIH) during the 1970s. "We wanted to have so much data that theconclusions from our study of this controversial issue could be veryrobust," Hedges comments. Earlier studies using many fewer geneswere disturbing to some researchers because they yielded a wide rangeof dates for the origin of animal species, although all the genestudies agreed that the event occurred well before the Cambrianperiod. "Our methodology and our larger data set should have had astabilizing effect; and in fact, our study resulted in a dateintermediate between the earlier estimates," Hedges says.

If the results of his team's genetic study are correct, Hedges saysthe scientific question must change from "How did all these speciesevolve so suddenly early in the Cambrian period?" to "Why don't wesee any fossils of these species long before the Cambrian period?" Among the suggested answers are that changes in the Earth'satmosphere led to the development of hard external skeletons inanimals that had only soft external skeletons before the Cambrianperiod. "Hard body parts like external skeletons are most likely tobecome fossils," Hedges explains. Species not likely to fossilize,like earthworms, typically live and die without leaving a trace oftheir existence--except in the genes of their descendants.

Another hypothesis is that many species of animals with skeletonswere living on Earth before the Cambrian period, but they were sosmall that their fossils have not yet been found. "The further backin time you want to look in the fossil record, the fewer places thereare on Earth to look," Hedges explains. Fossils have to be safelyencased in sedimentary rock, which, over time, melts or becomesdeformed by the movement of the Earth's crust. Sedimentary rocksover 3 billion years old are very rare. "If we can find very-old andvery-fine-grained phosphate sediments, which can preserve even softbodies, we might have the potential of finding fossils of these earlyanimals, even if they were only microscopic in size," Hedges says. "We seem to be missing the fossils of a lot of species."

Hedges says his research might be useful for finding life on otherplanets. "If we can learn when different stages of life evolved onEarth, we can compare those dates to events in the chemical evolutionof Earth's atmosphere and ocean, such as when oxygen and otherimportant gases increased," Hedges explains. Research with this goalis an important focus in Penn State's Astrobiology Research Center. "Our goal is to see if the early history of life on Earth can give usclues for how to predict life on other planets and in other solarsystems," Hedges says. "We hope to be able to predict the kinds oflifeforms that are likely to exist on other planets, based on thosethat existed during Earth's history, just by measuring the chemicalcontent of the planet's atmosphere."

This research was supported by grants from the National ScienceFoundation and the National Aeronautics and Space Administration.


NEAR's Eros Flyby Movie
MPEG Movie (2.3 MB)

This movie shows the asteroid Eros as seen from the Near EarthAsteroid Rendezvous (NEAR) spacecraft on December 23, 1998, when NEARflew within 2320 miles (3830 kilometers) of the asteroid. Eros, avery elongated, cratered object about 18 by 8 miles (30 by 14kilometers) across, is seen rotating with a period of just over 5hours.

The movie shows about two-thirds of a rotation of Eros. The firstview, taken at 10:44 AM EST from a range of 7150 miles (11,890 km),shows about half of the dayside of Eros (phase angle 87�). The movieends at 2:05 PM EST, just after closest approach, when only a tinyportion of the dayside of Eros is seen (phase angle 119�). During themovie, the spacecraft's view of the asteroid changed dramatically. Asis the case with most asteroids, Eros is rotating uniformly about afixed axis, and is not tumbling randomly through space.

A firing of NEAR's main engine at 5 PM EST December 20, designed tomatch the spacecraft's velocity with Eros's for insertion into orbitaround the asteroid, was aborted by the spacecraft. Contact withground controllers was temporarily lost, but was regained at 8 PM ESTDecember 21 when autonomous spacecraft safety protocols took over andtransmitted a signal to the ground. All spacecraft systems weredetermined to be healthy and operational. Within hours, a flybyobservation sequence was developed and uploaded to the spacecraft.1026 images were acquired by the multispectral imager, to determinethe size, shape, morphology, rotational state, and color propertiesof Eros, and to search for small moons. The infrared spectrometermeasured spectral properties of the asteroid to determine whatminerals are present, and the magnetometer searched for a naturalmagnetic field. Analysis of the spacecraft radio signal during theflyby yields bounds on the asteroid's mass and density.

The main engine was fired successfully on January 3, 1999, placingNEAR on-course for a February 2000 rendezvous. Eros is NEAR's secondasteroid encountered. On June 27, 1997, NEAR flew by the main-beltasteroid Mathilde at a range of 1212 kilometers (750 miles).





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