SYDNEY-Clearly, man is on his way to evolving back into a fish, and someday in the not-too-distant future, when our descendants once again all have gills, fins and a tail, the races will be held not in a pool but on a mudbank alongside it.

That has to be the only explanation for the, um, remarkable im-

provement in the performance of the swimmers at the 2000 Olympics.

In every event, at all distances, among both the men and the women, world records are going down like bricks dropped into a swimming pool.

By the end of Day 5, 12 world records and 18 Olympic records had been set or equaled.

Last night, Misty Hyman of the U.S., who had had a previously undistinguished career, broke the Olympic record by more than one second in winning the gold in the 200 meter butterfly. She just missed the world record, by a scant .07 of a second, and upset Aussie golden girl Susie O’Neill, the new Dawn Fraser.

Earlier, Lenny Krayzelburg of the U.S., who had already set an Olympic record in winning the 100 meter backstroke earlier in the week, set an Olympic record in the semifinal of the 200 meter back. The record he broke was one he had set the day before in a heat.

And in the feature event of the evening, Dutchman Pieter van den Hoogenband would have set a world record in winning the 100 meter freestyle if he hadn’t already swum a previously unthinkable time the day before, 47.74, breaking a world record set just minutes earlier by Australia’s Michael Klim in the other semifinal.

By comparison, “Hoogie’s” winning time – 48.30 – seemed positively crustacean.

It couldn’t be those futuristic – and at the same time, antique-looking – swimsuits, could it? The ones that cover up more skin than the ones worn by Mack Sennett’s Bathing Beauties back in the Roaring ’20s?

As far as we know, the nature and consistency of water hasn’t changed in the four years since Atlanta.

And perish the thought that the swimmers – or any other athlete here in Sydney with the exception of those cheating Romanian weightlifters – could be getting some chemical help from the friendly neighborhood team pharmacist.

After all, these are supposed to be the drug-free Games.

And, as Gary Hall Jr. rather pointedly observed last night after finishing third to van den Hoogenband and Alexander Popov, “You can’t accuse someone of taking drugs just because they swim fast.”

No, you certainly can’t.

But when everyone, it seems, is swimming fast, and in many cases, faster than they ever seemed capable of, you might be tempted to wonder.

American coach Richard Quick, for one, believes some Olympic swimmers are using drugs, and he criticized the quality and frequency of testing.

“I’m not pointing any fingers at anyone or any nation, but it’s definitely not drug-free,” he said. “The IOC should make that the No. 1 priority.”

Quick, the U.S. women’s coach, has been a vocal critic of the quality of drug testing in international swimming. Asked if he had proof of cheating, Quick said he was relying on his intuition.

“Look at the depth in many of the fields. A lot of great athletes are not in the finals and are not medaling,” he said. “I’m sorry that our sport has a cloud over these type of performances.”

Already, the aforementioned two Romanian weightlifters have been expelled from the Games for drug positives. A Bulgarian weightlifter was stripped of his silver medal for the same reason. A hammer thrower from Belarus was sent home, a Ukrainian shotputter has been suspended and so has a member of a Kenyan relay team. A dozen athletes from various nations have been warned not to bother showing up.

And just the other day, the general manager of the athletes’ village, a Mr. Maurice Holland, ordered 700 syringe bins – you know, the striking red ones you see in the doctor’s office for the disposal of “sharps” – to be placed in competitors rooms to collect what was termed “a glut” of used syringes.

The official line is that the athletes are using the syringes to shoot themselves up with “vitamins,” usually B-12, a perfectly legal practice that has never been shown to improve performance by a single microsecond.

And yet, a lot of people are doing a lot better than they ever have before in these Games.

Take the case of Inge de Bruijn of Netherlands, van den Hoogenband’s teammate.

She set a world record in winning the gold in the 100 meter butterfly. She also set one in a heat of the women’s 100 meter freestyle. At 26, after a decidedly ordinary career, de Bruijn has suddenly blossomed.

Already, people are whispering that de Bruijn is this year’s Michelle Smith, the Irish swimmer who four years ago made a sudden and dramatic improvement, won a fistful of gold in Atlanta, passed every test – and has since been banned for life for tampering with a urine specimen.