But not all of the pieces of the conventional wisdom fit together.

New analyses of the magnetism preserved in rocks suggested that the southern edge of Eurasia was farther north than some had thought, raising the question of whether India was close enough to make contact with Asia by then.

“In the old days, we thought that when India collided with southern Asia, southern Tibet was 2,000 kilometers south of where it is now,” said Peter H. Molnar, a professor of geological sciences at the University of Colorado. “And that all made sense. If you collide 40 to 50 million years ago, and southern Asia was down there, you have one collision, bang, you’re done.”

The revised magnetic measurements suggest that southern Tibet was 1,000 kilometers south at most. “Suddenly you have this huge space,” Dr. Molnar said. “Where was India 50 million years ago? Well, India was way south of where southern Asia was. So you have a choice: You have to make northern India be enormous or you have to put an ocean in there.”

Others pointed out that the most obvious effects of a continental collision, like the raising of the Tibetan plateau, did not start until later, perhaps 10 million years later. The rocks in the western Himalaya region also point to two collisions, not one.

“The jury was in,” said Simon L. Klemperer, a geophysicist at Stanford. “The jury is out again.”

Another key question: How much of India has disappeared? If the collision occurred more than 50 million years ago, almost 2,000 miles of India must have been pushed under Asia. A piece of continent this large would appear to be too big to fit the available space in ancient Gondwana.

And yet scientists agree that something collided 50 million to 55 million years ago, clearly seen in an uplifted seafloor of this age in the northern Himalaya region.

“That is why people said there is a collision between two continents,” Dr. van Hinsbergen said. “Which is true. That is the best conclusion you can draw. The question is not so much whether there is a continent colliding 50 million years ago. The question is whether that is India.”