Excerpts from New Scientist:
In evolution, as in life, some things are easier than others. It seems to be pretty straightforward to evolve complex eyes, which have turned up dozens of times.
Similarly, for some groups of animals it's easy to stop laying eggs and start giving birth to live young. Backboned animals have evolved live birth no fewer than 132 times, and nowadays a fifth of lizards and snakes give birth. Human mothers may disagree, but live birth is clearly not that difficult.
What is difficult, however, is nourishing unborn young the way mammals do. A female mammal allows each embryo to burrow deep into the wall of her womb, where it takes nutrients straight from her blood. This intimate arrangement was long thought to have only evolved once, in mammals.
Not so. It now appears that it evolved at least twice: once in mammals, and once in an obscure African lizard called Trachylepis ivensii. ...
All live-bearing reptiles have a basic placenta, but unlike its mammalian counterpart the embryo doesn't get much food that way. It can't: although it nestles up against the oviduct wall, the embryo remains inside a remnant of eggshell that acts as a barrier. Instead, it is nourished by a large yolk.
A very few reptiles, including T. ivensii, break this rule. Their eggs are small, with little yolk, so they must get lots of food from their mothers via the placenta. But only T. ivensii allows the embryo to implant itself in the oviduct wall. "It's unprecedented," Blackburn says. ...
But the arrangement has its problems, Blackburn says. An embryo in close contact with its mother's blood risks being attacked by her immune system. Male embryos could also be "feminised" by her sex hormones. That might explain why full-scale placental feeding has evolved so rarely.
The only reptiles that come close to T. ivensii belong to a South American skink called Mabuya. Their placentas are complex and transfer plenty of nutrients, and there is some evidence of embryonic cells being able to invade the oviduct wall in a limited way. Another African species, Eumecia anchietae, also feeds its young entirely through its placenta.
Neither shows as much intimacy as T. ivensii, however, and Blackburn also says their placentas are "fundamentally different". That may mean placental feeding has evolved in skinks not once, but three times. Clearly, in evolutionary terms these reptiles have the knack.
Neat, isn't it? Clearly, science contains just as much wonder as pseudoscience. Plus, it's true. And for me, that matters.
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