Think of a chicken egg. Sometimes you can see a tiny embryo amid the yolk and the egg white, which - if you weren't going to eat them scrambled - would be all the embryo needs to become a fluffy, yellow chick.

Now, consider the human placenta, an advanced version of the yolk and the egg white. It supports a growing baby for months, constantly delivering oxygen and nutrients from its mother. It is the only mammalian organ that grows in adulthood. It's also the only one with a natural expiration date, as it is expelled after childbirth.

The evolution of the human placenta remains something of a mystery to scientists. But a recently published study by Stanford University researchers offers some clues, suggesting that the placenta of humans and other mammals first evolved from the tissue clinging to the insides of eggshells.

Its findings, published online April 7 in the journal Genome Research, could one day prove useful in treating birth defects or pregnancy problems, several scientists not affiliated with the study said.

"We need to know how the placenta works because dysfunction of the placenta results in many complications of pregnancy," among them preeclampsia, placenta previa, and premature birth, said Derek Wildman, a Wayne State University evolutionary biologist affiliated with the Perinatology Research Branch of the National Institute of Child Health and Human Development. "This is the basic work that will allow us to rationally study those syndromes."

In the future, Wildman said, it's possible that genetic "biomarkers" indicating potential problems with the placenta could be identified through a simple blood test. Other researchers familiar with the study suggested that its findings also could help shed light on immune disorders and the development of cancerous tumors.

To conduct the study, Stanford geneticist Julie Baker and graduate student Kirstin Knox, examined the placentas of pregnant mice throughout their roughly 20-day pregnancies, studying which genes appeared to be active during the various phases of gestation. Then they used genomic databases to identify which of the genes were shared among species and which were specific to a species. Using a separate database, they also examined genes from full-term human placentas.

The researchers found that from the beginning to the middle of the mice's pregnancies, the placenta activated genes that are ancient and common to many species ranging from yeast to birds to reptiles. That, Baker said, suggested that the placenta evolved by tweaking 100-million-year-old genes.

"What it really tells us is that the hard-wiring for a placenta was there way, way back - in yeast, in reptiles."

Then, in the last half of pregnancy, the placenta's cells start "switching on" newer - say 80-million-year-old - genes that are species-specific: mice placentas activate mice genes, while human placentas activate human genes.

These species-specific genes appear to regulate physiological changes to the mother - telling mom more frequently that she's hungry, for example - and temporarily alter her immune system so that it doesn't reject the developing embryo as a foreign object, said Jay Cross, a professor of comparative biology at the University of Calgary.

Before she started studying the placenta, Baker was surprised to learn how little is known about the short-lived organ, even though "50 percent of the population has to deal with it," she said.

"Anything that we can understand about how it works, and what our ancestors can tell us about it, is really exciting."