Vertebrate embryos follow a very specific developmental pathway from single cells to entire organisms. Anna Keyte and Kathleen Smith of Duke University have found that marsupial embryos do not adhere to this strictly controlled timeline, but instead have limbs that jump the queue.
In all other vertebrates, the internal organs mature before the limbs appear. By the time the limbs have muscles or bones, the rest of the fetus is fairly well developed. Marsupials, however, are born at an early embryonic state and finish gestating in their mother’s pouch rather than in her uterus or in an egg like other vertebrates. The tiny creatures must make their way unassisted to their mothers’ pouches, which can be a considerable distance in some species. Thus, marsupial embryos require strong forelimbs far earlier than other vertebrates do.
An embryonic short-tailed opossum finishing its development outside the womb.
Credit: Anna KeyteKeyte and Smith found that the genes for limb expression were turned on much earlier in marsupials than in other vertebrates, and that more cells from the early embryos were slated for limb development. As you can see from the picture, these embryos are little more than slugs with weightlifters' arms. Once the task of getting into the pouch in completed, the embryos can continue their development like any other vertebrate.