February 22, 1981

LUCY The Beginnings of Humankind. By Donald C. Johanson and Maitland A. Edey. Illustrated. 409 pp. New York: Simon & Schuster. $16.95. THE story of human evolution - how apelike animals living millions of years ago changed into the human beings of today - is one of the great scientific epics of all time. But, like the sagas of the ancients, the details of the hominid odyssey have tended to vary with the teller. During the past 20 years, and especially during the past 10, significant rewritings have appeared; unfortunately for anyone seeking a clear interpretation of human evolution, most have been based as much on changing opinion and ideology as on new evidence. ''Lucy: The Beginnings of Humankind'' is, therefore, a welcome and needed corrective. Dr. Donald Johanson is a paleoanthropologist, based at the Cleveland Museum of Natural History, whose fossil discoveries since 1973 have forced a major reinterpretation of the early stages of human evolution. Maitland Edey is a first-rate science writer and the author of the excellent and popular volume ''Early Man.'' Together, in clear engaging prose, they trace the rocky history of efforts over the past century to find and interpret the most significant hominid fossils, while at the same time they tell of Dr. Johanson's own remarkable fossil discoveries in Ethiopia.

''The missing and murdered children'' is spoken as a single word here in a distraught city frustrated by the baffling, unsolved deaths of 18 children and the mysterious disappearances of two others in the past 19 months. Two new names, one of a dead child and one of a missing boy, swelled the list to 20 black victims yesterday while city officials were saying that they were conducting the most extensive investigation in the nation's history. Yet they say they are no closer to arresting suspects than they were when an investigative task force began work last summer. So far, the investigators have tried everything from filming funeral services to consulting with psychics. Citizens have formed mass search parties to look for victims and have established groups to seek children under the age of 17 who are violating the 7 P.M. citywide curfew. City firefighters have walked door-to-door asking for scraps of information and distributing pamphlets on child safety. Agents of the Federal Bureau of Investigation have come from around the country, forming teams each day to scour woods and deserted areas for victims.

Mark Aguirre scored 24 points tonight but he might have easily made it 30 or more as he led De Paul to its 10th straight victory and 23d of the season by beating Marquette, 78-71, at the Horizon. ''We had to tell him to shoot,'' said Ray Meyer, the Blue Demons' coach. ''Sometimes it's frustrating when Mark starts passing off so much. He gave up a couple of easy 15-foot shots out there. But then, we won, didn't we? And a wise old man once told me, 'You can't do any better than win.' ''

The Economics of Hope Some private economists are skeptical of the Reagan Administration's inflation projections, which seem to be more of a hope than a forecast. According to the President's economists, tax cuts, reductions in government spending and tight monetary policy will bring inflation down from 11 percent this year to just over 8 percent in 1982, and to 6.2 percent the year after. By 1986, Mr. Reagan forecasts that the Consumer Price Index will show an annual increase of 4.2 percent. Economists like Lawrence Chimerine of Chase Econometrics and Michael Evans of Evans Economics feel the President is overly optimistic. While both men expect inflation to drop over the next five years, they doubt that productivity, which they feel is the key to lower inflation, will improve as rapidly as the President expects. Moreover, they point out that wage increases, which have been running behind the rate of inflation, may average 12 percent for the next few years, further damaging the outlook. Finally, the Reagan forecast seems to assume stability in the world petroleum market, a luxury that disappeared in the early 1970's.

WHITE PLAINS ALTHOUGH the murder trial of Jean S. Harris has been the longest in recent memory in Westchester County, law enforcement officials say, it has not had an adverse impact on the judicial system here, which was already overburdened when pretrial hearings in the case began last Oct. 6. For more than four months, the trial consumed the efforts of Judge Russell R. Leggett and his staff, plus a Clerk and Court Reporter. ''This has been one of the longest trials in the county's history,'' said a spokesman for Justice Joseph F. Gagliardi, Administrative Judge for the Ninth Judicial District. ''But, it has not delayed other cases. We have not been affected adversely by the Harris trial.''