Saturday, November 28, 2009

Fragments, Figments and Visionaries

When a fossil is found it is often broken up into small fragments. Sometimes, one small fragment is all that is found of a once whole skeleton, from a once whole individual, which lived and breathed and maybe reproduced and certainly died. On rare and serendipitous occasions, entire skulls or skeletons are found. But even then, depending on the circumstances of preservation, parts of the entire skeleton may be broken into smithereens.

Paleontologists are looking to reconstruct the past. After fossils are collected, the paleontologists all drift to sleep in their tents and dream about what this individual looked like in life, or what types of substrates it climbed on, or if it had stripes or speckles, or neither. But in the morning light, with a cool objective head, they snap out of it. Debuting threads of their visions only in slightly drunken, partial jokes at the camp table. They must just be absolutely burning to know what extinct species were like. Wanting, secretly and desperately to catch one impossible glimpse of it in life. And ultimately, isnt it that fantasy, fueled by the persistent mystery of the past, that engenders curiosity in all the historical sciences? It has to be. The fossil discovery is not the inspirational and orderly end of a scientific story, its the wild and unruly beginning.

In order to draw meaningful conclusions about the crumbly former animal the first thing that is often done is that the pieces are sometimes literally glued together. In the case of hominin fossils, and especially of hominin crania, casts of the original fossil are made and plaster fills in the spaces where the fossil is missing. The problem with this approach is that often these plaster filled structures, which are like hardened inferences, are used in analyses without regard for much of the inherent uncertainty of their form. For example, sometimes there are several possible orientations between two fragments, but they may be glued one way and firmly thought of that way for years to come.

More recently, with the advent of digital imaging, all the fragments of a hominin crania can be CT scanned and then manipulated in a virtual environment. The missing pieces can be inferred given biological and statistical prior probabilities, instead of just filled in with plaster. Taphonomic distortion can be corrected. Also, several possible reconstructions can be experimented with, without damaging the original fossil. In this case, the reconstruction can become one possible evolutionary hypothesis, just the way a phylogenetic tree is. When building phylogenetic trees, there is a method of measuring how well the data supports each given branch, its called bootstrapping. I wonder if it would be possible to ascertain a sort of bootstrap value for the position of each cranial fragment relative to the other neighboring fragments. The bootstrap value is based on resampling the genetic dataset again and again. The equivalent would be sampling of positions of the fossil fragments again and again. Anyway, there are many testable simulations that are opened up with this approach to reconstruction.

Another aspect of fossil reconstruction, which is done primarily for the purpose of popular science, is adding musculature and hypothetical skin, hair and eyes on fossil forms. When musculature is inferred, modern analogies are used to determine their direction, position and robusticity. This is not unlike looking at a fossil bone and determining its function based on modern analogies of how extant taxa use this bone. These fully fleshed out creatures can then be used in museums, documentaries, magazines etc. They present a hypothesis of how the extinct individual may have looked, based on the given data, and what can be ascertained via extant analogy. In the case of hominins especially, these reconstructions are almost always uncomfortable and goofy to look at, but its not necessarily because they are inaccurate, its because we have never seen anything like it before, and never will.

Scientists sometimes scoff at these physical or virtual forms, skeptical of the plaster, or the amount of silly scienceless speculation that is poured into each structure. These forms are sometimes squarely dismissed as “highly reconstructed”, or even, art. Its easy to critique a visual inference as unsupported conjecture. In fact, its too easy. I dont see what the alternative is to reconstructing a very fragmented fossil. For example, put a pile of fossil fragments in front of any paleontologist and then tie their hands behind their back and ask them what they are looking at. Free their hands and watch them pick it up, turn it around, inspect its morphology and see how it all fits together. A pile of fragments does not contribute to science in the same way an attempt at reconstructing it does. Sure, the conclusions drawn from this structure, that began as a pile, could be appropriately cautious, but you simply need to somehow put it together first. No one could resist.

I know what science is, and what it isnt. I am not suggesting that we build fantastical features in the space of missing bones, just that we acknowledge the skill, intent and biological information that do go into making reconstructions. Like many things, if its built only in your mind and not with your hands, its less vulnerable to hasty, dismissive critique. Where would paleontologists be without artists reconstructions anyway, to realize their visions, in fact or in error? Its not shameful to admit that we all have elaborate evolutionary fantasies. And I think some of the swift discrediting of this type of work stems from the fact that what scientists have imagined, in their tent, is not what materializes. But surely they have imagined something.

1 comment:

  1. Sometimes I wish I were a better artist so that I could be one of the few who get to see their imagined reconstructions realized by their own hands.