Remember our favorite 50,000-year-old cave person? You know, the one who gave us "fight or flight," the one who taught our bio-systems to run away from tigers because the boss is glowering? Well, it turns out he's making us fat, too.

Dr. Henry A. Jordan, who is one of this country's gurus of weight-control-a-la-behavior modification, feels that we have a biological bias toward overeating.

And why? Well, because back 50,000 years ago, when we were running away from real tigers or fighting on a rock-stick-spear level, we also got into the habit of stuffing ourselves whenever food was around, because who knew when another lucky rock would down another edible whatever. And the most efficient and safest method to store energy?

You got it -- fat!

But, as with the flight-or-fight response, what passes for civilization -- in this country, anyway -- once again outpaced human adaptability. That primitive deep inside us never heard of salt, never mind refrigeration.

So everything else being equal, Dr. Jordan's biological bias tells us to stuff our faces whenever we can. "Man," he says, "is basically predisposed to eat, to consume food if it is in our environment . . .Of cource this is an enormously adaptive response in cultures where food supplies are scarce . . ."

(Similarly, he believes we have a predisposition to conserve calories by being as inactive as we can, saving up -- our internal primitive ancestor is telling us -- for the time when we have to get up and get out.)

It isn't hard to see, then, how these coupled predispositions contribute to our overweight. After all, when was the last time you were threatened by a tiger? And isn't food "in our environment" more or less constantly?

Dr. Jordan, a practicing psychiatrist (in Philadelphia), who is svelte (but smokes a lot), has been into eating for 20 years or so. When he says something like "there's always room for dessert," he's not just telling you something you've suspected all along, even if you have. He's done the studies to prove "that the mouth drives the system." (Suggestion: Don't save the best for last. Eat it first!)

In the past decade or so, his principal food orientation has been toward weight control. He is concerned that the overall failure rate of all diets, diet systems, fads, fasts or anything else is a depressing 95 percent. Maybe even more.

Those primitive biases are part of the problem, he believes, along with:

The conviction in our culture -- carefully instilled by the obese -- that fat people have no will power, no capability to succeed at anything, especially a diet.

The basic requirement, at the same time, by most diets that the dieter be perfect, a virtual guarantee of failure.

The use of food in our society, quite possibly having its origin in the afterschool snack, as a transition from one part of the day to another. Dr. Jordan calls it "the use of food in time management."

"We anchor our day, essentially, around meal times," he points out, "so that it gives us an opportunity to take breaks. We have a coffee break, we have a lunch break. It is not a acceptable, somehow, just to take a 15-minute break in the middle of the morning, but it is acceptable if you ingest food.

"Students do it: It's not acceptable just to leave the books for 10 minutes, but it is if you're hungry. It's not acceptable just to put your head on your desk for 10 minutes. It should be.

"And, this is a biggie, we use food to alleviate boredom -- a very simple, easy thing to do." Or to kill what he calls "an unexpected piece of time," say at an airport when a plane is late. (Pace up and down, he suggests, or browse in a bookstore.)

In an approach he calls "cognitive behaviorism," Dr. Jordan is trying to outwit the primitive saboteur, undo the unfair and self-defeating image of the fat person as slothful and incapable, eliminate the concept basic to so many diets that food is poison, and to make sure the dieter plans his or her favorite but no longer sinful things as part of the diet.

Jordan, who is, among other things, director of the Institute for Behavioral Education, in King of Prussia, Pa., uses groups and behavioral techniques, but emphasizes the importance of individualized therapy. "The behavioral rules," he says, "are not etched in stone . . . a rule useful for one (patient) may not be relevant at all to the next.

"One thing I've learned is that every time a new patient walks in I'll find a new use of food, or a new combination of uses . . ."

To deal with the time-management problem he suggests substitutes like taking a bath, changing clothes, lying down going for a walk.

His latest study, financed by Campbell Soups, is likely to turn up one of these days in ads promoting soups for dieting.

Soup, as it turns out, is indeed one of those foods particularly useful in lowering caloric intake by changing eating habits. The study was conducted by Dr. Jordan and colleagues at the University of Pennsylvania's Wharton Applied Research Center among 517 dieting volunteers in the Greater Philadelphia area. It found that dieters who ate soup for lunch ingested 54.5 calories less per lunch than those who did not. Also, that on days when soup was eaten for lunch, the daily intake was lower by an average of 92 calories.

"Soup," says Dr. Jordan, "is a model for other foods that slow down the rate of ingestion." And that speeds up the effects of a behavior-modification oriented diet.

Other "slow-down foods" are those that:

Need to be eaten with a utensil -- soups, of course, or, for example, stew, rather than a hamburger.

Are very hot -- baked, rather than French-fried potatoes.

Need to be peeled or shelled -- crabs and lobsters, shelled nuts (rather than out of a jar), a whole orange, rather than a gulped glass of orange juice. t

Most of all, feels Dr. Jordan, things that are eaten slowly can be tasted, savored, enjoyed. "Use food," he recommends, "as an ally."