Feb
18
2010

Digging Up Our Family Tree

I am the instructor for an online course for teachers titled “Teaching Evolution”. When I surveyed these teachers this past fall about some of the most common misconceptions their students have with respect to evolution, they all chimed in – “humans come from monkeys”.

This should probably not be too surprising, given that (1) children are taught to confuse monkeys and chimps from an early age – Curious George is not a monkey, he has no tail and is therefore an ape – and yet, repeated throughout the stories is the phrase “Curious George was a monkey”, (2) human evolution does not appear in state standards in many states (see Mead and Mates. 2009. Why science standards are important to a strong science curriculum and how states measure up.  Evo Edu Outreach 2:359-371) and is therefore often not taught; (3) even good educational programs like NOVA can, unfortunately, perpetuate part of the misconception. Don’t get me wrong, I love NOVA. Their Judgment Day documentary about the Kitzmiller v. Dover trial is great. I just watched What Darwin Didn’t Know and I love how they explained the importance of similarities in embryos as evidence for evolution, among other excellent points made in the film. And overall, I liked the Becoming Human series. However, it has drawn some criticisms worth revisiting as we look back on 2009, a year when the description of the fossil hominin Ardipithecus ramidus was hailed as the most important science discovery of the year. An accurate understanding of human evolution does matter.

Unfortunately, the NOVA Becoming Human program, in attempting to explain the evolutionary transition between our hominin ancestor and present day humans, constantly draws a comparison between modern day chimpanzees and Homo sapiens, which is really an incorrect comparison to make, and fuels the misconception that humans evolved from “monkeys”.  For example, numerous times statements like “[m]illions of years ago, we were apes, living ape lives in Africa” are paired with video segments of modern day chimpanzees and gorillas, which unfortunately promotes the misconception that we evolved from modern day chimpanzees, or even monkeys, since I’m guessing many people do not readily distinguish between chimps and monkeys.

[A quick digression: Why use of hominin instead of hominid? Hominini is a tribe of Homininae that comprises humans, chimpanzees, bonobos, their ancestors, and the extinct lineages of their common ancestor. Members of the tribe are called hominins. Hominids, taxonomically Hominidae, form a family that includes humans, chimpanzees, bonobos, gorillas and orangutans and the extinct lineages of their common ancestor. Hence when speaking of our ancestor with chimps, given the current taxonomy, we should use hominin.]

The assumption behind the human-chimp comparison is, of course, that our hominin ancestor would have looked much like modern chimps, and herein lies the problem. Chimps have their own evolutionary history. Yes, they are our closest living relatives, but looking to chimps for information about our hominin ancestor is about as informative as looking to my third cousin for information about my great-great grandmother. I can certainly hypothesize that traits found in both my third cousin and myself were present in our common ancestor, my great-great grandmother, but each of us also have our own evolutionary history and I certainly wouldn’t assume that my great great-grandmother looked exactly like my third cousin. We only have to look to Darwin for clarification of this point “[i]n the first place it should always be borne in mind what sort of intermediate forms must, on my theory, have formerly existed. I have found it difficult, when looking at any two species, to avoid picturing to myself, forms directly intermediate between them. But this is a wholly false view; we should always look for forms intermediate between each species and a common but unknown progenitor; and the progenitor will generally have differed in some respects from all its modified descendants.” (Darwin 1859 Chapt. 9)

The Becoming Human series should have made explicit why they were making such a comparison, since the search for shared derived characters or synapomorphies is actually the foundation of cladistics and much of modern evolutionary biology. There are a number of shared derived characters which indicate our shared ancestry with chimps, and which separate apes from other primates. These traits include: relatively large brain, absence of tail, more erect posture, greater flexibility of hips and ankles, increased flexibility of wrist and thumbs, and changes in structures of the arms and shoulder. However, each lineage (one leading to chimps and another to modern day humans) also accumulated different traits, and the shared derived traits along the direct lineage to humans, the lineage that begins since we shared a common ancestor with chimps, informs our understanding of where, when, and how we became modern humans.

The most appropriate comparisons are with our actual ancestors, for which we now have many examples.   Here is another issue with the NOVA program: it did not include the most recent discovery of Ardipithecus ramidus. We can excuse this omission on grounds that Science had yet to publish the most current findings; however, Ardipithecus ramidus had been known of for 15 years. Could the Becoming Human series have waited a few more months? Without discussion of Ardipithecus ramidus, the best depiction of the earliest fossils of the tribe Hominina comes from Sahelanthropus tchadensis. However, only a skull, five pieces of jaw and some teeth comprise what we know about this species. It is not clear whether Sahelanthropus tchadensis walked upright. Evidence from Ardipithecus ramidus, however, suggests that by 4 million years ago, our  ancestors did walk upright, and that our upright posture evolved long before our brain capacity increased.

By the end of the three part program, however, I was less stressed over the perpetuation of the chimp to human comparison, and more excited by some of the newer findings presented in the series – for example, that extreme fluctuations in climate were probably associated with the array of evolutionary changes taking place, and that we can tell we inherited the FoxP2 gene from our common ancestor with Homo neanderthalensis -work by Svante Paabo indicates the gene is the same in both groups. The picture of a group of individuals, described as Homo heidelbergensis, migrating north into Europe, giving rise to Homo neanderthalensis and a separate group being pushed to the edges of Africa, giving rise to Homo sapiens, was also enlightening. Here again, however, what helped visualize human evolution was the inclusion of trees – phylogenetic hypotheses explaining our current understanding of the evolutionary relationships among various extinct and extant species. Again, these are hypotheses, which will undoubtedly be altered as new fossils are discovered, and new technological advances allow us to gain more insight into our genetic connection to chimpanzees and perhaps even our hominin ancestors. In light of the tentative and useful nature of trees, I’ve included one here, showing some of the more significant synapomorphies for various clades of primates, along with the current names for these groups.

Family-tree-cladogram

I also found the additional resources provided for teachers at the NOVA Becoming Human website to be very useful.

Resources on NOVA site

http://www.pbs.org/wgbh/nova/beta/evolution/our-family-tree.html

http://www.pbs.org/wgbh/nova/beta/evolution/compare-skeletons.html

http://www.pbs.org/wgbh/nova/beta/evolution/zoo-you.html

Written by louisemead in: Biology Teaching |

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