One organ, arising some 120 million years ago, stands above all others in the mammal鈥檚 evolutionary ascent. The placenta, it now appears, evolved by hijacking ancient genes found in nearly all forms of life.
But the huge differences in mammalian gestation 鈥� a mouse carries more than a dozen pups for 20 days, while an elephant delivers one calf after 22 months 鈥� might be due to recent evolution, according to an analysis of the genes involved in the growth of the placenta.
There are few genes shared between distantly related mammals, says , a developmental biologist at Stanford University in California, who led the study.
Advertisement
鈥淭he things that sustain a pregnancy in a mouse are going to be different than the things that sustain a pregnancy in a whale,鈥� she says.
Something borrowed聟
Without the placenta, mammals wouldn鈥檛 exist. The nutrient-rich tissue nourishes a developing animal throughout its time in the mother鈥檚 womb, and then becomes useless after birth.
Despite its importance, the organ remains an evolutionary black box. 鈥淭here鈥檚 very little known about where the placenta came from,鈥� Baker says.
To illuminate the placenta鈥檚 history, Baker and her colleague Kirstin Knox studied the organ鈥檚 development in mice.
At several intervals during mouse pregnancy, the researchers took genetic snapshots of the placenta that measured which genes were active. Baker鈥檚 team then looked at the evolutionary history of the genes turned on by the growing placenta.
Mid-term switch
Researchers can do this by comparing the DNA sequences of mouse genes to those found in other animals and even yeast and bacteria. Genes similar to versions found in other organisms are almost certainly ancient, while genes unique to mice must be recent additions.
When Baker鈥檚 team did this, they found the placenta had led a double life. When the organ starts growing, it switches on ancient genes shared by all animals and even yeast 鈥� many of them linked to cell growth and division.
Midway through pregnancy, however, the placenta activates an entirely new set of genes, many found only in mice. 鈥淚t鈥檚 a completely different beast at day 15 than it was at day 10, even though, morphologically, it looks identical,鈥� Baker says.
Post-term human placentas activate lots of primate-specific genes, her team also discovered.
Different demands
Turning on newly evolved, animal-specific genes just before pregnancy might explain why vastly different mammals make do with the same organ, Baker says.
鈥淒ifferent genes had to be created to come up with different demands on the fetus,鈥� she says. 鈥淵ou鈥檝e got animals that live in water, animals that live in air, animals that live on land. Some are giant, some are tiny.鈥�
, a biologist at Wayne State University in Detroit, agrees with that appraisal, but he wonders whether the trend holds for other mammals besides mice and humans. 鈥淚t would be cool to look at a whale or dolphin or monkey or horse,鈥� he says.
Journal reference:
Evolution 鈥� Learn more about the struggle to survive in our comprehensive special report.
Genetics 鈥� Keep up with the pace in our continually updated special report.
