What was going on when I was born?

JENNY O'BRIEN and Ben Cotton attend a preparatory school, yet they are hardly typical preppies. Neither has any intention of applying to Yale, Harvard or Princeton. Their goal is to enter the United States Military Academy at West Point. Like their classmates at the Military Academy Preparatory School at Fort Monmouth, N.J., they wear Army fatiques to class rather than the Brooks Brothers shirts de rigueur at other exclusive schools. Applications for admission to the military academies have risen steadily in the last few years, giving the Fort Monmouth school, along with the Naval Academy Preparatory School in Newport, R.I., and the Air Force Preparatory School in Colorado Springs, all quasimilitary institutions operated by the Defense Department, an increasingly important role in preparing the nation's future military leaders.

The professional politicians will be watching the early returns in Indiana, Maine, Rhode Island, New Jersey and Virginia tonight, seeking the first clues on how the Democrats are doing in their efforts to win the mid-term election. Those states will provide the initial indications of whether Democrats are benefiting from either an unusually small or an unusually large turnout; whether the Republican base vote is active, disaffected or staying home; whether blacks are coming out in force to protest Reagan Republicanism, or whether Republicans are the ones showing surprising strength. Even before the polls close, the political experts will have diagnosed the weather. Good weather in California, central Illinois, Florida or Delaware could help Democrats increase their turnout. The three major television networks will have conducted surveys with voters leaving the ballot booths to help project trends and winners in important battlegrounds like New York, New Jersey, Texas and California.

million-year-old grave in South Africa, had belonged to an animal the size of a ferret, about a foot long. This was one species, Thrinaxodon liorhinus, of the therapsids that dominated terrestrial life for millions of years before the rise of dinosaurs. Not only did the dinosaurs overwhelm the therapsids in nature's continuing war of survival, driving them to extinction, but they also have overshadowed them in the minds of humans who essay to reconstruct this distant past. The neglect is undeserved. For just before their kind vanished, some therapsids made the fateful transition from reptiles to the first mammals. They were the ancestors of every bat and cat, whale and primate, every human being. There in the little Thrinaxodon jaw, Dr. Crompton pointed out, was one of the more telling clues to the evolutionary trend that would be the therapsids' destiny. The teeth-bearing bone, the dentary, was clearly developing a nonreptilian size and shape. Other jaw bones were already shifting toward their future places in the mammalian middle ear.

A list of candidates in Metropolitan Report yesterday incorrectly identified one contender in the 84th Assembly District. He is J. Henry Neale Jr., running on the Democrtic and Liberal lines.

French-Manitobans win their case A2 Soviet bars pacificist group from holding news conference A3 Attorney General Smith ''heart- ened'' by visit to Thailand A4 Around the World A5 U.S. backing raids on Nicaraguan border A6 Three members of hospital staff testify before massacre panel A13 Government/Politics Cuomo appeals to his supporters to get out the vote B1 City must answer charges by striking Legal Aid lawyers B3 Senate candidates in Jersey seek votes of independents B4 Challengers in Connecticut contests continue campaigning B4 Reagan shows concern on Social Security as election issue D22 ELECTION DAY Poll hours - New York, 6 A.M. to 9 P.M.; New Jersey, 7 A.M. to 8 P.M.; Connecticut, 6 A.M. to 8 P.M. Parking - Alternate-side rules suspended. Sanitation - No collections; no street cleaning. Stock and commodity markets, and banks - Some open, some closed; see page D2. HO Federal offices - Open. State and municipal offices - New York and New Jersey, closed; Connecticut, open. Post offices - Regular operations. Transportation - Subways, buses, Long Island Rail Road and Conrail on regular weekday schedules. Libraries - Closed. Schools - New York, closed; New Jersey and Connecticut, closings vary.

A year and a half after its maiden flight, People Express Airlines is still aloft and even turning a profit in what has proved to be a dream-shattering atmosphere for some of its competitors. At the outset of this era of deregulation, it looked easy. The general formula was lower-cost labor coupled with used aircraft to produce lower fares. But some upstarts failed to get financing, some went broke and others are struggling with deficits. The recession, the air controllers' strike and competition from the established lines combined to make the task formidable.

Second article of a series appearing periodically on the new balance of power in the Middle East since the war in Lebanon. BEIRUT, Lebanon, Oct. 28 - The Israeli invasion of Lebanon did more than simply devastate the Palestine Liberation Organization militarily. It turned Lebanese politics upside down, tipping the balance of power radically in favor of the Maronite Christian minority and leaving the Moslem majority submissive and disillusioned. In many ways the events of last summer constituted Lebanon's third civil war since independence, with a predominantly Christian portion of the population either passively welcoming or openly cooperating with the Israelis during the invasion, and a predominantly Moslem segment putting up whatever resistance it could muster. Unlike previous civil wars in Lebanon, however, this one did not end with the traditional formula of ''no victor, no vanquished.'' The Israelis did not come to Lebanon to supervise a stalemate. Rather, there was a very clear victor, the Christians, as represented by the Phalangist militia and party, and a very clear vanquished, the Moslem left, as represented by the coalition known as the National Movement, which had bet its all on Syria and the P.L.O.

Today is Election Day. The polls will be open from 6 A.M. to 9 P.M. in New York State, from 7 A.M. to 8 P.M. in New Jersey and from 6 A.M. to 8 P.M. in Connecticut. These are the principal contests: New York State - Voters will select a governor, lieutenant governor, state attorney general, state comptroller, a United States senator, all 34 United States representatives, all members of the State Senate and Assembly and many judges and local officials, including 35 members of the City Council in New York City. There will be ballot questions in New York City, Nassau County and Suffolk County. Yesterday, Lieut. Gov. Mario M. Cuomo, the Democratic-Liberal candidate for governor, campaigned in the city yesterday, and Lewis E. Lehrman, the Republican-Conservative candidate, traveled across the state. Page B1.

Westchester County and the Marriott Corporation have agreed that Marriott will end its role as manager of Playland, the amusement park in Rye, next summer. The corporation's two-year contract to operate the 52-year-old county park on Long Island Sound will end on Dec. 31. Marriott incurred a $2.6 million operating deficit in its first year and a reported deficit of about $3 million in 1982. County Executive Alfred B. DelBello said in a letter sent today to the County Board of Legislators that a professional general manager for the park would be recruited ''on a one-year-contract basis'' and that Marriott had offered to help in the search. The move, Mr. Del-Bello wrote, would allow the board to ''concentrate on developing a plan for the future.''

AMERICAN psychiatrists are increasingly distressed because they believe too few medical school students are being attracted to the profession to keep psychiatry vital and to meet the nation's mental health needs. Some psychiatrists conclude that the decade-long plunge in interest that led to the current situation reflects a disillusionment on the part of medical students over the scientific validity and practical effectiveness of the discipline. They also point to student dissatisfaction with a prospective income that has become relatively low in comparison to that of other physicians. From 1970 to 1980 the percentage of medical students drawn to psychiatry fell from above 11 percent to less than half that proportion. That dramatic decline has become a force compelling a profession that often encourages introspection in its patients to engage in some intensive self-scrutiny of its own. As a result of this critical evaluation, efforts are under way to raise the scientific quality of psychiatric education in medical schools, to promote recruitment into the field and to improve the public's esteem for psychiatry.

Because of an error by campaign aides, a chart in Metropolitan Report yesterday on the New Jersey race for United States Senator incorrectly reported the Middle East position of Frank R. Lautenberg, the Democratic candidate. He does not favor the creation of an independent Palestinian homeland; he favors a territory federated with Jordan.

President Reagan has agreed to expand the mission of the United States Marines in Lebanon to include patrols in East Beirut, the State Department said today. The move was taken at the request of President Amin Gemayel of Lebanon who wants the American-Italian-French force to assume more important responsibilities, Administration officials said. A Defense Department spokesman said that the new duties for the marines would be in addition to maintaining security at the Beirut International Airport, where they have been stationed since arriving more than a month ago, and would not necessitate any increase in the size of the 1,200-man contingent. Gemayel Seeks 30,000-Man Force Mr. Gemayel has said publicly that he would like to see the 3,400-man American-French-Italian force in the Beirut area expanded to about 30,000, to help insure the withdrawal from Lebanon of all Israeli, Syrian and Palestine Liberation Organization forces.