fossils

An end to Ediacaran embryology?

/ zcofran / 5 Comments

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, 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.

ResearchBlogging.orgNeat 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

eFfing Fossil Friday (another late edition)

/ zcofran / 2 Comments
ResearchBlogging.orgI’m sitting at a cafe in Tbilisi, departing at 4:00 am tomorrow for America. Readers will notice that I’ve been MIA while working with the second annual Dmanisi Paleoanthropology Field School. I hate to say it but I’m glad I was too busy to blog all the goings-on (though sorry if it disappointed anyone). All in all it was another great year, and we found some great fossils (about which I don’t think I have permission to say anything at all). Here’s this year’s class with their certification of badassery at the site on the last day:
But Dmanisi won’t be the subject of this belated eFfing Fossil Friday. I’d like instead to turn to the question of just what fossils are good for. I’m told that in China, fossil teeth were once interpreted as dragons’ teeth, and so pulverized and sold as medicine. But what good are they to non-medical science? My recent research interests have come to focus on the relationship between evolution and development. Evolutionary developmental biology (“evo-devo”) research has been dominated by studies of genes, gene expression, and model organisms like fruit flies and mice. In such an environment, the question of the relevance of fossils is especially poignant.
But this morning, while planning a human evo-devo course I hope to teach next summer, I stumbled upon a review paper by Rudolf Raff, titled “Written in Stone: Fossils, genes and evo-devo” (2007). I think the abstract sums things up pretty well:

Fossils give evo-devo a past. They inform phylogenetic trees to show the direction of evolution of developmental features, and they can reveal ancient body plans. Fossils also provide the primary data that are used to date past events, including divergence times needed to estimate molecular clocks, which provide rates of developmental evolution. Fossils can set boundaries for hypotheses that are generated from living developmental systems, and for predictions of ancestral development and morphologies. Finally, although fossils rarely yield data on developmental processes directly, informative examples occur of extraordinary preservation of soft body parts, embryos and genomic information.

It seems often that fossils are falling by the wayside. There’s a sentiment that there’s not much information to be gotten from fossils – they’re too incomplete, too few, too inconvenient, at least as compared with extremely high-output data such as that coming from genomics. But Raff is right – we need fossils. Beyond the excellent points Raff raises in the review, I’m working on getting the most out of these seemingly data-poor fossil samples. Because modern computers are so powerful nowadays, I’m using their sheer processing power to test hypotheses about growth and development in fossil samples. These battered bunches of bones are too tiny to be analyzed by traditional methods. But one thing I think is important to take away from this computer-crazy Information Age, is that we now have machines that can handle almost any kind of question one can think to ask, and it’s really inspiring. The sequencing and analyses of ancient Neandertal and Denisova genomes (Green et al. 2010, Reich et al. 2010) are excellent examples of the amazing research that can be done with computers and creativity (and probably also a horde of hard-working math majors).
So this eFFF (or Sunday) is not dedicated to any specific fossil or set of fossils, but rather to all fossils, even the crappy fragments. Gaumarjos, fossils: your secrets are not safe from us.
Reference
Green, R., Krause, J., Briggs, A., Maricic, T., Stenzel, U., Kircher, M., Patterson, N., Li, H., Zhai, W., Fritz, M., Hansen, N., Durand, E., Malaspinas, A., Jensen, J., Marques-Bonet, T., Alkan, C., Prufer, K., Meyer, M., Burbano, H., Good, J., Schultz, R., Aximu-Petri, A., Butthof, A., Hober, B., Hoffner, B., Siegemund, M., Weihmann, A., Nusbaum, C., Lander, E., Russ, C., Novod, N., Affourtit, J., Egholm, M., Verna, C., Rudan, P., Brajkovic, D., Kucan, Z., Gusic, I., Doronichev, V., Golovanova, L., Lalueza-Fox, C., de la Rasilla, M., Fortea, J., Rosas, A., Schmitz, R., Johnson, P., Eichler, E., Falush, D., Birney, E., Mullikin, J., Slatkin, M., Nielsen, R., Kelso, J., Lachmann, M., Reich, D., & Paabo, S. (2010). A Draft Sequence of the Neandertal Genome Science, 328 (5979), 710-722 DOI: 10.1126/science.1188021


Raff, R. (2007). Written in stone: fossils, genes and evo–devo Nature Reviews Genetics, 8 (12), 911-920 DOI: 10.1038/nrg2225
Reich D, Green RE, Kircher M, Krause J, Patterson N, Durand EY, Viola B, Briggs AW, Stenzel U, Johnson PL, Maricic T, Good JM, Marques-Bonet T, Alkan C, Fu Q, Mallick S, Li H, Meyer M, Eichler EE, Stoneking M, Richards M, Talamo S, Shunkov MV, Derevianko AP, Hublin JJ, Kelso J, Slatkin M, & Pääbo S (2010). Genetic history of an archaic hominin group from Denisova Cave in Siberia. Nature, 468 (7327), 1053-60 PMID: 21179161

eFFING FOSSIL FRIDAYS!

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I’m going to do my best to keep up with the blog during by Big Summer Adventure, and one thing I’d like to do is “F-ing Fossil Friday!” in which I focus on fossils for a bit. We’ll see if I can make this pan out.
Today I got out the rest of the Australopithecus robustus mandibles at the Transvaal Museum (above), save for I think maybe 1. As you can see from the picture, taphonomy (what happens to an animal’s remains between death and our digging them up) creates a serious challenge for the study of variation in this species. I’m focusing on ontogenetic variation – differences associated with growth and development. In spite of its fragmentary nature, so far as I know this is the best ontogenetic series of any fossil hominid (I should probably look more into A. afarensis here, too). In the bottom left you’ll see SK 438, the youngest in the sample, whose baby teeth haven’t quite come in all the way. Poor little guy! At the top right corner is SK 12, probably the oldest individual and also a big bugger.
One thing that I’ve noticed so far, only a preliminary observation that I need to actually run some numbers on, is that as individuals get older, the length of their tooth row (molars and premolars) gets shorter. This is because of the tendency for teeth to move forward during growth – “mesial drift” – and for adjacent teeth to literally wear into one another, their ends becoming flatter and flatter. While I should have realized this, it was surprising at first to find some dimensions of the lower jaw actually decreasing during growth. Now, I still have to run some tests to see if this is a biologically significant phenomenon. But it’s always nice to learn something new, even after just 2 days back with my best extinct buddies.
Stay tuned to future eFfing fossil Fridays!