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It entered the water at the same place it had emerged — it saw its own tracks in the sand — and, staying in the shelter of the boulders, made its way slowly down the sloping mud bottom until it was immersed up to its shoulders.

It emptied its lungs of air, ducked underwater and, as its brain told it to do, generated motion in its gill flaps, opened its mouth, expanded its trachea and breathed in.

It choked.

It sprang instantly to the surface, gasping and coughing.  Pain seared its lungs and knotted the muscles in its abdomen.

Enervated and off balance, it slipped and began to sink.  Water seeped into its gill slits, and again it choked and gagged.  It reached for an outcropping on one of the boulders, grabbed it and clung, wheezing, until at last its lungs were clear.

Twice more it tried to submerge, following each step of the ancient program.  Twice more it failed.

It did not know what had happened, or why, for its brain could not ask itself such questions and thus could provide no answers.  It knew only that it could no longer exist underwater, that survival depended on breathing air.

But it also sensed that it could not survive among the air-breathing things.

If it could not live underwater, still it would have to live in water.

It drew a breath of air, clamped its gill slits closed and ducked down.  This time it did not choke.  It could see, for the lenses surrounding its eyes were intact, and it could move.  Tentatively, it swam forward.

But when it attempted to dive, it noticed a difference:  diving was no longer simple, fluid, natural; diving had become difficult, and a pressure within drew it up toward the air.

There was another difference; very quickly its lungs began to ache, there was a pounding in its ears, and its brain commanded it to find air to breathe.

It arced upward, broke through the surface and gasped.  As it breathed in and out, its buoyancy changed, and it had to kick slowly to maintain its position.

Its simple brain was challenged.  The changes required adaptations if it was to survive.

After a few moments, it felt comfortable enough to swim gradually away from shore.  Across the water it saw land.

Staying underwater as long as it could, surfacing only to breathe, it swam toward the land.  There, it sensed, it could find safety.

There it could hunt.