Nov 20 2007
Lucy, You Got Some ‘Splainin’ To Do
This is the conclusion of my interview with Dirk Van Tuerenhout, Ph.D., Curator of Anthropology at the Houston Museum of Natural Science. The first half of the interview is titled I Love Lucy.
Dirk, here’s a question from an associate; he mentions that creationists usually say that none of the other hominin species we know about today have anything to do with humans. According to them, fossils like Lucy are not the remains of a related species, let alone a direct human ancestor, but merely “some ape gone extinct”. So how do scientists arrive at the genealogical trees we all know from biology text books?
This is a question that requires an extensive answer. This question has shades of a favorite creationist line to critique/attack on Darwinian evolution. “Darwin says we all came from monkeys. Can you believe that?!”
Here is what we can say to that:
- Darwin never said we evolved from monkeys.
- However, we do share with other apes a common ancestor (ultimately also with monkeys, but much more remotely in time).
- We are as humans also part of the family of Great Apes.
In order to ascertain what links might exist between us and Lucy, we need to define what constitutes a modern human and then see which traits if any we can find with Lucy. Scientists have identified four traits that define a modern human:
- Walking upright all the time (a.k.a. habitual bipedalism)
- Making and using tools (complex tools especially)
- Having a large brain compared to one’s body size
- Having the ability to communicate (especially complex communication)
Having these traits make us modern humans human. When, however, do we see these traits appear in the fossil record?
Take Lucy, at 3.18 million years ago. She only had one of these traits: she walked upright all the time. In other words, Lucy, like us, was a habitual bipedal creature. That is a significant trait to have and one that separates her from the other apes (living or extinct), because they did not.
Moreover, and equally important: genetic data help us make that distinction more clear cut as well: based on comparing DNA from modern human beings and that of our closest non-human primate relatives (a complicated way of saying: chimps, gorillas and orangutans) we differ very little with these guys. The genetic difference between us and a chimp is about 1% in our DNA. Scientists have calculated that it may have taken about 7 million years for that 1% difference to accumulate over time.
While this number is certainly subject to revision and further study (nothing is black and white in science, there is a continuous questioning of what we know), based on what we know today we can say this:
- Lucy is a habitual bipedal creature (we know this because of her hips, hip and knee joints) and so are we.
- Lucy dates to 3.18 million years ago. This postdates the split between our lineage and that of the other Great Apes by about 3 to 4 million years. So in that regard Lucy also belongs on our side of the tracks.
In other words, both her bones and modern DNA data make Lucy a distant relative of ours and not one of the extinct non-human apes.
So how does one put a family tree together?
Initially the family tree was put together based on observed similarities between organisms. Specifically, this led to the development of a family of Hominoidea, of which we are the only member as this time and the family of the Pongidae in which the chimps, gorillas and orangutans are placed. This classification, or taxonomy, goes back to that devised by Linnaeus. He classified based on what he could see.
Nowadays, we classify also based on genetics. That has resulted in the coning of the term hominin, rather than hominid, in which both humans and chimps are placed together (because they are so closely related genetically), with the gorillas and orangutans left in the remaining niche.
In image form, this is what we are talking about:
The traditional view first, based on observed similarities:
Here chimp, gorilla and orangutans are categorized together, because they look a lot more like each other and not that much like a human.
The more recent classification next, based on genetic relationships:
How do we know that the various hominin species are related and, more importantly, how do we know in what way they are related to each other?
How do we determine the degree of relatedness among extinct hominins? While it is a human trait to want to see everything classified and neatly placed in a pigeon hole, it is not always possible to do this – at least not right away.We always work from the known toward the unknown. At this stage, when a fossil is found, it is compared against known fossils. “Known” here means that we know where they are from, how old they are and where they are placed in the family tree. A new fossil also has a location where it was found and eventually will also have a date assigned to it. What remains (far from simple) is to determine where your fossil fits in. Again you have to work with the known, or in this case: what did you find? If you have a complete skull, it might be much easier to make this determination than if you have found a foot or wrist bone. You compare morphologies and suggest degrees of similarities (and therefore old fashioned taxonomic relationships – see above) to establish a place in the family tree.
Remember, however: science will continue to subject any finding to future insights which might require revisiting and fine-tuning previous conclusions.
When we want to assign a fossil a spot in the genealogical tree, do we have to rely on inferences from bone morphology alone or are there other means of supporting our model?
Given that most fossils (with the exception of H. sapiens and Neanderthalers) do not have extractable DNA in them, we have to rely on morphology, place of discovery and dates to assign a place on the family tree (or family bush as we are calling it today).
What, on the other hand, would we expect to find under the creationists’ hypothesis?
The way in which this is phrased is too kind to creationism: hypothesis belongs in a scientific framework. Creationism or its “scientific” clone, I.D., does not work with hypotheses – no matter how much they would like to disagree with this. In the end, they already have their conclusion: an intelligent creator, or God made everything and here is the evidence for that line of thinking. Creationists/I.D. people are very good at casting doubts on scientist’s interpretations, producing a prodigious amount of materials everywhere to disseminate this perception that scientists do not really know what they are talking about. In reality what they are picking up on is the debate inherent in science on the manner in which evolution has proceeded in the past, not a discussion about the existence of evolution itself. Small detail with important repercussions.
In order to make the point that creationists do not really work with scientific hypotheses but that they prefer to cast aspersions on other people’s thinking to push their agenda, I refer to a point raised earlier: “creationists say Lucy is not an extinct human ancestor but rather an extinct ape.”
By framing the discussion a priori they have – in their mind – already won the battle. They have effectively removed Lucy from our lineage and so more doubt and scorn can be heaped on any arguments to the contrary. One needs to go back to the very crux of the matter and prove that their assertion of where Lucy belongs is wrong in the first place. Having reconstituted a level playing field, then you can proceed by proving that she is indeed an early human ancestor.
As best as I can tell then, a Creationist could not care less about what these fossils might look like and where they belong. In the end, they all see them as evidence of a creation by a Creator to confuse us or to make us marvel at the creation we live in. This is not a form of logic I can follow, but that seems to be their line.
Dirk Van Tuerenhout leverages his time to provide educational opportunities about the Lucy fossil to museum visitors while also teaching at the University of Houston-Clear Lake during the summer. If you are in the Houston area, I encourage you to visit the Houston Museum of Natural Science, and of course, Lucy.
Further Reading
-old fossil. We prepared our children with the expectation that Mom and Dad would be “taking a very long time” reviewing the entire exhibit. And we did. The exhibit is incredible.
