Every natural history and anthropological museum in the world seems to have a skeletal replica of Lucy, our 3.2 million year-old human predecessor. The discovery of her bones and close study of their shapes have provided a crucial observation regarding human development. Lucy proves that our ancient ancestors first stood up and walked, and only later did they develop large brains. This was a monumental observation, widely discussed.
My monumental observation, kept to myself until now, did not require any close study but did lead to unsettling questions. Five ribs, not twelve? Only one finger and one toe? Half a pelvis? Lucy, how could you walk with less then one leg?
I can imagine the experts, snickering at my naïveté, puffing up and proudly exclaiming that what we have is 40% Lucy. “Amazingly intact.” “An extraordinary specimen, fully sufficient to determine sex, brain size, and walking posture.” “A priceless morsel of world heritage.” Submissively I would have to respond, “OK. I get it, but as a rough approximation, let’s agree to call what’s on display Lu. I want to know what happened to cy.”
The science of taphonomy (Greek for burial + law) attempts to answer such queries. What happens to a newly dead plant or animal until it disappears or until its fossil is discovered? Simply put for bones, how did these end up here and where are the rest?
Bones exposed to sunlight and air decompose just as soft tissues do, only much more slowly. The moisture and fat are gone in a year or two. Surface crackling ensues and are followed by flaking and fissures, which deepen. Eventually the bone, once so solid and proud, splinters into brittle bits. This takes between six and fifteen years depending on temperature, humidity, and size of the animal.
Exposed bones, however, are rarely left alone. Hyenas swallow whole bones and may travel considerable distance before depositing them. Ravens take sheep bones into their nests. Otters may carry fish onto stone outcroppings hundreds of feet above the water and leave the bones there. Ice and flowing water move bones down hill, and wind can roll small, particularly roundish, bones in any direction. Abrasion and polishing occur by wave and tidal action and can simulate human use. Gnaw marks indicate that a small animal was satisfying its calcium needs. Hack marks imply that man was satisfying his protein needs. Trampling can break the bones, and taphonomists can tell by the configuration of a fracture whether it occurred during life, at the time of death, or later.
Buried bones do not fare all that well either. Soil compaction flattens skulls and rib cages and can cause long bone fractures, for instance when one leg crosses over the other and the lower one becomes a fulcrum. Soil temperature, moisture content, and acidity all play their parts in degradation. Moisture content and pH in particular can differ considerably over short distances, which may expose part of a skeleton to one set of conditions that is not experienced by the rest.
It gets worse. Buried bones can become re-exposed or shift. Movement includes everything from tectonic plate upheaval and mud and ice slides to human exhumation and reburial. Trampling can not only break bones, it can also churn them into disorder with bones from other animals. Also stepping directly on a bone can push it down into older layers of sediment. Conversely, stepping next to a bone may squish it closer to the surface and thereby lead the erroneous conclusion that it was more recently deposited. When a skeleton is found relatively intact and in a life-like position, the taphonomists conclude that the body was silted over shortly after death and was sheltered from movement.
How do paleontologists (fossil fans) and anthropologists (human fans) find bones for the taphonomists to ponder their course through time? There are three major ways.
The first is to go where the money is–excavate at known or likely sites. These include the Olduvai Gorge in Tanzania, the La Brea Tar Pits in Los Angeles, and cemeteries and Indian mounds everywhere. The second is to search areas subject to erosion and exposure, such as river banks, retreating glaciers, and especially the ancient sea beds that once covered the midwestern United States. Paleontologists are known to walk for miles along exposed faces of sedimentary rock with their eyes glued to the ground looking for bone. What had been completely hidden last season may have become exposed by winter weather erosion. The third is serendipity. Contractors likely hate it when their excavations are halted while the science nerds descend into building foundation sites, utility trenches, and highway cuts to salvage priceless markers of the ancient past.
Lu is one such priceless marker. Lucy showed up through a combination of planning and luck. A paleontologist suspected that one particular area of Ethiopia was a likely repository for remnants of human origins. So he mounted an expedition. During the second season of exploration, a gully that had been searched twice before was almost overlooked the third time before a fragment of Lu’s arm bone caught the paleontologist’s eye. The rest is history.
Well almost. Consider that fossils (stone bones) weather just as stones and calcium bones do. Once exposed, what may have been buried for eons can turn to pebbles and powder in a few years. We are lucky that Lucy’s body was silted over shortly after her demise and that it remained undisturbed for the next 3.2 million years. We are again lucky that it was exposed at a time when a trained eye happened to spot it.
Now back to my original question. Lucy, where are the rest of your bones? It is possible that the cy portions of her skeleton were exposed previously and eroded or were washed away. Or maybe cy fell prey to other taphonomic processes. The fact that we have Lu to document an important step in human development is astounding and wonderful. Five years earlier, Lucy may have been completely concealed. A year or two after Lu’s discovery, a paleontologist looking at the same spot might have found only L. Or nothing.