A stomach would seem a very useful organ for any animal that eats big meals. Yet over the years - probably many， many millions of years - a large number animals have lost their stomachs. Examples range from the platypus and ghost shark to the lungfish. A team of scientists wanted to probe how such animals evolved to live without these meal digesting organs. Their first step： They looked at the animals' genes.

The researchers focused on 14 various verte-brates - animals with backbones - comparing the genes in those with and without stomachs. They found a distinct difference between them. No stomach-free animal had genes to produce an acid-rich zone in its digestive tract. Animals without stomachs also lacked genes to produce pepsin.

Animals without stomachs still digest their meals - just in a different way. The rest of their plumbing， for instance， may do extra work.

True stomachs evolved in vertebrates. Some 450 million years ago， glands that make food-breakdown chemicals began developing in what would become stomachs. Since then， such glands have been lost at least 15 separate times across the animal tree of life.

Today， more than one in every four bony fishes digests its food without a true acid-making stomach. Picking out what drove this evolutionary change is tricky. Some no-stomach animals eat meat. Some just eat plants. And certainly， what any of them eats now could be different from what its ancestors ate when their stomachs first began their disappearing act， millions of years ago.