Q. Do all living things share the same basic DNA? What determines that something will turn out a jellyfish or a human?

Beverly

A. According to Walter Gilbert, a Harvard molecular biologist who shared the 1980 Nobel prize for his work on DNA, everything from bacteria up, with the exception of some viruses, shares some similar DNA.

There are many genes, in many species, that create proteins that have the same biochemical function. In some cases, those genes are identical; sometimes they're very similary. That leads us to believe that all living things probably trace back to a single ancestral DNA, he says.

The divergence that led in one direction to single-celled bacteria and in another to humans probably occurred 4 billion years ago, though some think it was only 2 billion years ago, Gilbert says.

People and fish probably diverged from a a common ancestor about 400 million years ago. People and cows, as recently as 150 million years ago.

But while many human genes are similar to those in other life forms, each species has evolved enough unique genes to distinguish itself on life's family tree. Birds evolved genes for wings. Fish evolved genes for gills. Humans evolved genes for golf.

One big DNA difference among species is quantity. Our genome is 3 billion chemical units long and has about 100,000 genes. E. coli bacteria have about 3,000 genes. And along a strand of human DNA, while sections called exons turn into genes that make proteins we need, other sections, called introns, don't code for genes at all. Scientists aren't sure what introns do.

Simple organisms have less waste. The DNA of the tiny worm C. elegans is only 100 million units long and is almost all exons.

There are obvious genetic differences even within species. In humans, DNA disparity determines gender, race, or susceptibility to certain diseases. People differ from each other by about one tenth of 1 percent. We differ from our closest genetic relatives, chimpanzees, by about 2 percent, Gilbert says.

So why don't we look 98 percent like a chimpanzee? Gilbert says that even small differences in genotype (an organism's genetic code), can translate into big differences in phenotype (the physical form the organism takes).

Once development gets started, the hormones and tissues and organs and proteins in the embryo, plus the environment it's in, start to interact with each other and with our DNA as the organism develops.

Genes are recipes, or prescriptions, for doing things, he says. They're almost never absolute statements. The environment shapes how they end up doing their job.

Gilbert says we understand the molecular biology of DNA a lot better than we do the developmental biology of how we get from genotype to phenotype. He says we'll have to understand that process much better before we can turn what we know about DNA into genetic engineering techniques that can grow new human organs or eliminate disease.