Roughly 600 million years ago, thousands of embryos of primitive animals drifted into seawater laced with sulfides and died. But in addition to potentially killing them, the sulfides protected the microscopic nascent creatures and permitted the process of fossilization to occur. Research in the Doushantuo Formation in China uncovered these fossilized embryos--among the rarest of finds both for their fragile nature and depth in past time--and new imaging techniques have provided a window into the internal workings of the most ancient animals yet discovered.
Whitey Hagadorn of Amherst College led an interdisciplinary and international team of scientists that isolated 162 of the most well-preserved specimens. Each showed evidence of an intact fertilization envelope--the surrounding membrane that kept the dividing cells together--as a change in the encompassing rock. "Compositional variation tends to correspond to fossil boundaries," Hagadorn notes. "The embryos are intact. They are not deflated or deformed." This provides strong evidence that the embryos have not been influenced by either the decay associated with death or subsequent geologic processes. Using x-ray computed tomography as well as scanning and transmission electron microscopy, the scientists then peered inside these ancient animals and created three-dimensional models of their innards.
The models revealed intricate structures inside the cells, including a lack of stiff cell walls as are typical in the fossilized remnants of fungi and algae. "The cell membranes of the embryos are a lot more flexible," explains Shuhai Xiao, a geologist at Virginia Tech and a team member, also noting that embryos fossilized in the act of division show no increase in volume. "Putting all this evidence together, it suggests these embryos have some animal features and are probably allied with animals closer than anything else." And these oldest animal embryos already show some of the hallmarks of animal embryo development, such as cells dividing at different rates and specialized internal structures. "These old-time embryos also had membrane-bound vesicles inside them containing things that would be used in development," says team member Rudolf Raff, an evolutionary development biologist at Indiana University. "Furthermore, the fact that some of the cells are not dividing at the same rate as other cells looks like many kinds of living embryos." Such asynchronous cell division allows modern animals to relatively quickly create specialized cells for different organs, such as the gut or skin.
But the embryos also show their age, lacking structures such as skin, or epithelium cells common among even the most primitive animals, such as sponges. In another example, even the largest fossilized embryos found lacked a blastocoel, a fluid-filled hole in the embryo's center that is common to most living animal embryos. "Either these embryos are primitive and don't have a clear blastocoel, or a blastocoel existed but didn't survive the preservation process," Raff notes. And despite the astonishing similarity to modern embryos, the possibility remains that geologic processes could have resulted in rock formations that simply mimic the shape of animal embryos. "In the Cambrian time [roughly 542 million years ago] we have better evidence, the preservation style is broadly similar," Xiao says. "Preservation and morphology are similar and that makes us confident it's not diagenetic."
Although the scientists are reasonably sure the fossilized embryos are real, according to the paper presenting the research in the October 13 issue of Science, they have no idea what kind of animals these embryos represent. "What we think we have is a relatively primitive stem group metazoan," Hagadorn says. "But the next obvious question is not just what these embryos are but how many different kinds of animals are represented. We're trying to distinguish between soccer balls and volleyballs." That process would be helped immensely by finding fossils of the juvenile stages of these creatures, as is more common in the Cambrian era. "Getting larvae, which would be a dead giveaway, is hard because they don't seem to preserve very well," Raff laments. Then again, ancient embryos are not common either.