Preemies from the Precambrian
|Charles Darwin noted that even when adult animals of different species look distinctly different, the similar appearance of their embryos could reveal how they are related, and how they descended from a common ancestor.
Credit: John Collier (portrait) and James Stanfield (photo)
In his 1859 classic, On the Origin of Species, Charles Darwin noted that even when adult animals of different species look distinctly different, the similar appearance of their embryos could reveal how they are related, and how they descended from a common ancestor.
Now, almost a century and a half later, scientists are using a new technique called microCT – a spin-off of the medical CAT scan – to study rare fossils of the early-stage embryos of what may be Earth’s oldest animals.
While the fossils examined so far are no bigger than 32-cell embryos, they suggest the earliest stages of animal development haven’t changed much in 600 million years. Once more mature embryo fossils are found, researchers hope microCT will shed light on evolution of Earth’s earliest animals and, ultimately, on how the development of embryos evolved over time.
"By looking at these fossils, we’re trying to identify who were the first animals swimming around in the ocean or sitting on the seafloor, what environments were they living in – shallow marine settings or the deep sea – and what did they eat," says Whitey Hagadorn, an assistant professor of geology at Amherst College in Massachusetts.
Hagadorn used microCT to study fossils collected during the past five years from the Doushantuo Formation in southwest China. The rock contained what appear to be fossils of ancient algae and early-stage embryos of unknown animals.
|"They are possibly the oldest fossils of animals on Earth." -Paleontologist David Bottjer on the Doushantuo Formation fossils
Credit: University of Southern California
Recent dating, based on the decay of one lead isotope to another, puts the fossils’ age at 599 million years, give or take 4.2 million years, which would make them the oldest known animal fossils, Bottjer says.
"If you don’t look at them real close, they look like little round balls," Hagadorn says. The fossil embryos range from 100 to 500 microns in diameter (a hair is about 50 to 75 microns wide). Algae found in the same formation are roughly one-tenth as wide.
Hagadorn says he and his collaborator, Shuhai Xiao of Virginia Tech, have identified nine different forms or "morphotypes" among the dozens of animal embryo fossils they examined. These included embryos comprised of 1, 2, 4, 8, 16 and 32 cells, plus others with envelopes or coatings that were thin or thick. Hagadorn and Xiao don’t know if the nine types of embryos represent one or more species of early animal.
"There could be many taxa here that all develop in the same way, and we might not recognize it," Hagadorn says.
Hagadorn says fossilized embryos are "incredibly rare," and were preserved because their organic matter was replaced by calcium phosphate, which has tiny crystals that are not big enough to destroy fine structures such as individual cells.
|"[The discovery of ancient fossilized embryos] demonstrates that remarkably unpreservable material can be preserved." -Bruce Runnegar, NAI Director
The discovery of ancient fossilized embryos "demonstrates that remarkably unpreservable material can be preserved," says Bruce Runnegar, a professor of paleontology at the University of California, Los Angeles, and director of the NASA Astrobiology Institute. "The last thing you’d expect to find in the fossil record 500 million years in age or more are embryos."
Runnegar and Bottjer both note that with a maximum of 32 cells, there is only so much information to be obtained from the embryos Hagadorn examined.
"It’s a little difficult to do the compete embryology using what’s been discovered," Runnegar says. "Of course it’s really exciting there are any [fossil] embryos and that we know anything about them at all."
Yet, "what is really remarkable is that these fossilized embryos seem to be very similar to modern invertebrate embryos," says Bottjer, a professor of earth and biological sciences who once was Hagadorn’s adviser at USC. "They certainly tell us that how you start to build an animal hasn’t changed a whole hell of a lot since animals evolved," even if adult forms of animals diverged considerably.
Bottjer suspects that "even though they are the oldest fossils, there was probably a lot of evolution that went on beforehand, because the embryos already have a lot of features of modern embryos."
In the future, researchers want to use microCT on more mature fossil embryos to study how embryonic development changed over time.
"Evolutionary and developmental biology have begun to make predictions about how and when ancient animals diverged from one another," says Hagadorn. "One of the ways they are thought to have diverged is through different developmental pathways. So what does a many-celled embryo do differently that causes it to change into a sponge versus a coral? We have potential way to test that by looking at different stages represented in ancient embryos."
"Whitey’s [microCT] method is going to be able to give us a detailed 3-D description of these embryos and any others we find," Bottjer says.
|More recently, paleontologists have used room-sized medical CT (computerized tomography) scan equipment to X-ray fossils such as this dinosaur egg.
Credit: University of Glasgow
"It’s going to be very useful," Runnegar says. "We can see the three-dimensional geometry [of fossilized embryos] fairly easily in a nondestructive way."
He says that for more than a century, paleontologists have sectioned fossils and drawn a series of pictures of the cross-sections to construct three-dimensional images. More recently, paleontologists have used room-sized medical CT (computerized tomography) scan equipment to X-ray fossils such as dinosaur skulls, making three-dimensional images from a series of "slices" to get an idea of the internal structure, or to locate the fossil within rock.
MicrofocusCT or microCT, uses the same technology as medical CAT scans but in a version that first was miniaturized to look for microscopic defects in manufactured products such as aircraft components.
Hagadorn, working with geologist Shuhai Xiao of Virginia Tech in Blacksburg, Virginia, pioneered the use of desktop microCT equipment to examine the internal structure of ancient embryo fossils. Hagadorn started using microCT while doing postdoctoral research at NASA’s Jet Propulsion Laboratory (JPL), which was studying the technique for possible use on future Mars rover vehicles. JPL’s tabletop device measures roughly 2.5 feet wide by 5 feet long and is attached to a computer that combines X-ray cross sections into three-dimensional images.
Before the advent of microCT, the embryo fossils had been examined with other methods. Scanning electron microscopes (SEM) showed individual cells within the embryos, and could distinguish them from algal cells. Cells within an animal embryo deformed each other more than the stiffer plant cells in algae.
Conventional and petrographic microscopes were used to examine thin sections shaved off embryo fossils. They revealed internal structures inside the embryos, including cell membranes and dark spots that some scientists have suggested might be cell organelles, Hagadorn says.
|A SEM image depicting a suggested metazoan embryo – possibly Olivooides multisulcatus – at approximately the 256-cell stage.
He says microCT revealed even more about the embryos and the cells within them, including the way they are packed together to form hexagonal, pentagonal and other multifaceted faces typical of modern animal embryo cells.
In the four-celled embryos, "we saw paired ovoidal structures with a nucleus-like shape – like seeing seeds within the grapes," says Hagadorn. "There are two of them in each cell. We really don’t know what they are," although he wonders if the twin ovoids might be cell nuclei fossilized in the process of dividing.
In addition to conducting more microCT studies of the primitive embryos, Hagadorn wants to confirm or rule out whether their cells’ internal structures are cell nuclei.
"We look forward to scanning a lot more samples and investigating these embryos in detail," he says. "Maybe two years down the line someone will find another deposit [with fossils] that we can use this technique to look at."
Hagadorn wants to search for embryo fossils in slightly younger rock formations in China and Russia, and see how they changed from their Doushantuo ancestors.
Bottjer says paleontologists have scratched only the surface in extracting animal embryos and other fossils from the Doushanto. "There could be all sorts of interesting things to be discovered," he says.