An end to Ediacaran embryology?

The things people can do these days. Therese Huldtgren and colleagues reported in last week's Science that they identified nucleus-like structures in 570 million year old fossilized cells from China. These date to the Ediacaran period, before the "Cambrian explosion" of animal life forms. Superficially, these fossilized balls of cells rather resemble the early stages of animal embryos (see A in the figure below), in which cells are dividing and increasing in number but the overall embryo size stays the same. To get the "inside story" (...sorry), Huldtgren and team used very fancy "synchrotron x-ray computed tomography" to look at the insides of these fossilized cells. The resulting images have micrometer resolution - that's one thousandth* of a millimeter. The things people can do these days.

Fig. 2 from Huldtgren et al. 2011

And lo! each of these fossilized cells contains a small, globular structure that looks like a nucleus (left; if you cross your eyes you can merge the 2 halves of fig. C to make it look even more 3D).

Could these really be the earliest animal embryos? Probably not - some of these balls-of-cells had what resemble budding spores, unlike animals but similar to "nonmetazoan [non-animal] holozoans." In other words, something neat and old, but not one of our earliest ancestors.

I'm really impressed with the biological applications of computed tomography (CT). Recall that a while ago, I posted about the potential to use synchrotron tomography to examine the small-scale, internal structure of bone (e.g. Cooper et al. 2011). Such non-destructive, high-resolution imaging techniques could be used to compare near-cellular-level growth in living and fossil animals. This is really significant because it adds a completely new kind of information we can get from fossils, which before now could only be studied well at the gross, macroscopic level (though scanning electron microscopy of teeth has been very informative about diet; see for example Ungar and Sponheimer 2011). Indeed, one of the most common applications of CT imaging in anthropology is making 3D computer models of body parts for morphometric (shape) analysis.

But high-resolution, synchrotron CT imaging opens up a whole new world of paleontology, new questions that can be asked. For example, many researchers have examined the microscopic appearance of bone surfaces to determine whether bone was being added or removed during growth, and comparing different species (Bromage 1989, O'Higgins et al. 2001, McCollum 2008, Martinez-Mata et al. 2010). These have been very informative studies, but it is not totally clear how growth at the cellular level relates to growth at visible level. Moreover, fossil surfaces are often abraded, obfuscating surface details. So, I can envision using synchrotron microscopy similar to Cooper et al. (2011) and Huldtgren et al. (2011), to examine bone growth in fossil hominids, at and beneath the surface. This can help us understand how facial growth was modified over the course of human evolution, from the snouty visage of Australopithecus afarensis to the tiny, starry-eyed faces we have today. People could also examine how activities like chewing, running or even talking affect (and effect) bone growth. There is much work to be done.

Neat as these projects would be, it's pretty humbling to consider that we have the technology to analyze microscopic fossils hundreds of millions of years old, and shed light on the developmental processes in our earliest ancestors.

Read those things I'd mentioned

BROMAGE, T. (1989). Ontogeny of the early hominid face Journal of Human Evolution, 18 (8), 751-773 DOI: 10.1016/0047-2484(89)90088-2

Cooper, D., Erickson, B., Peele, A., Hannah, K., Thomas, C., & Clement, J. (2011). Visualization of 3D osteon morphology by synchrotron radiation micro-CT Journal of Anatomy, 219 (4), 481-489 DOI: 10.1111/j.1469-7580.2011.01398.x

Huldtgren, T., Cunningham, J., Yin, C., Stampanoni, M., Marone, F., Donoghue, P., & Bengtson, S. (2011). Fossilized Nuclei and Germination Structures Identify Ediacaran "Animal Embryos" as Encysting Protists Science, 334 (6063), 1696-1699 DOI: 10.1126/science.1209537

Martinez-Maza, C., Rosas, A., & Nieto-Diaz, M. (2010). Brief communication: Identification of bone formation and resorption surfaces by reflected light microscopy American Journal of Physical Anthropology, 143 (2), 313-320 DOI: 10.1002/ajpa.21352

McCollum, M. (2008). Nasomaxillary remodeling and facial form in robust Australopithecus: a reassessment Journal of Human Evolution, 54 (1), 2-14 DOI: 10.1016/j.jhevol.2007.05.013

O'Higgins, P., Chadfield, P., & Jones, N. (2001). Facial growth and the ontogeny of morphological variation within and between the primates Cebus apella and Cercocebus torquatus Journal of Zoology, 254 (3), 337-357 DOI: 10.1017/S095283690100084X

Ungar, P., & Sponheimer, M. (2011). The Diets of Early Hominins Science, 334 (6053), 190-193 DOI: 10.1126/science.1207701