Scientific Method

When a paleoanthropologist finds a fossil (Figure 1.2), she applies the scientific method to decipher its place in evolutionary history. Based on prior knowledge gained from other scientists' work and from her own observations, she forms a hypothesis. Then she collects evidence, or data, to test her hypothesis. This is the step where a chemist would perform an experiment, but a paleoanthropologist, instead, collects evidence of evolution's experiment by analyzing other fossils and the bones of living species that are similar.

If the data she collects supports her hypothesis, then the paleoanthro-pologist and other researchers repeat the scientific process to determine if additional evidence can support the same hypothesis. If it does, and

Figure 1.2 This photograph of the right side of a mandible, or lower jaw, of the extinct stem ape Proconsul was taken moments after its discovery. It was found peeking out of the ground at site R106 on Rusinga Island, Kenya, in the summer of 2006. For orientation, the molar teeth are on the left and they are heavily worn away from poor preservation. Photograph by Holly Dunsworth.

Figure 1.2 This photograph of the right side of a mandible, or lower jaw, of the extinct stem ape Proconsul was taken moments after its discovery. It was found peeking out of the ground at site R106 on Rusinga Island, Kenya, in the summer of 2006. For orientation, the molar teeth are on the left and they are heavily worn away from poor preservation. Photograph by Holly Dunsworth.

if the hypothesis continues to be supported, then that hypothesis turns into a theory. The stronger the support is for the theory, the stronger the evidence against it must be to falsify it. As it gains support in the form of repeated testing that theory will eventually become so strong that it will become part of the prior knowledge that other scientists apply to the formation of new hypotheses. The cycle continues endlessly with hypotheses being tested and either supported or not, with theories being overturned by the discovery of evidence that refutes them, with entirely new hypotheses being born when theories crumble, and with some theories standing the test of time.

Before collecting any fossils, a paleoanthropologist selects a field site based on the types of fossils that she can use to test her hypotheses. So, if her hypotheses are centered on the split between the chimpanzee and human lineages, she surveys a site with a known geologic age or one that corresponds in age to other sites that have already produced very early hominin fossils. She searches for rocks that match the age of genetic time estimates for the split around 6 Mya (see discussion of "Molecular Clocks" in Chapter 4).

Along with any fossils of chimpanzee or human ancestors, the pale-oanthropologist must also observe and collect the other types of fossils at the site. The preserved plant and animal remains will help reconstruct the ancient environment. The nature of the rock layers will also indicate whether a lake, a delta, a river, a gully, an animal burrow, a den, or many other types of burial scenarios deposited the sediment. (Hominins are not found inside intentionally dug graves until very recently.) These types of environmental reconstructions will put the species into an environmental context and will also provide additional information about ape and human evolution that the fossil cannot provide itself.

In the laboratory, the paleoanthropologist will analyze the anatomy of the fossils by recording measurements (e.g., the lengths and widths of the bones and teeth) and by looking inside the bones with imaging technology like x-ray computed tomography (i.e., "CT" or "cat" scanning; (Figure 1.3)). These data are then compared with other fossils and to skeletons of modern apes and humans.

The paleoanthropologist will test the hypothesis that the fossil she collected is more closely related to chimpanzees or to humans by testing if the anatomical traits are more like one than the other. If the fossil's characteristics are more like modern chimpanzees than humans, then it will be placed in the evolutionary lineage of chimpanzees. This placement is a hypothesis that can be overturned with additional evidence. After all, it is possible that an ape-like creature living 6 Mya was neither an ancestor of chimpanzees or humans because it may have belonged to a lineage that went extinct and did not contribute to our evolution or to that of our ape cousins.

The overarching framework in the quest for human origins is a theory that has achieved factual status: evolution by natural selection. The scientifically rigorous investigation of human origins and evolution was able to flourish after Charles Darwin made this contribution, partly because he provided hypotheses to test. For instance, in The Descent of Man (1871), Darwin postulated that fossils of the last common ancestor (LCA) of great apes and humans would be found in Africa since gorillas, bonobos, and chimpanzees, which are the most humanlike animals, currently live there. According to the current fossil record, Darwin's prediction is still correct, and, of course, he could be proved wrong if scientists find the evidence. It is unlikely, but possible nonetheless, that

Figure 1.3 A high-resolution virtual slice through a human skull (left) and a chimpanzee skull (right) at the temporal bone near the ear. The spongelike bone is the inside of the mastoid process which is the palpable bump just behind the ear that attaches muscles from the collarbone to the head. These are not fossilized, but many times fossils will still contain much of the infrastructure of bone, like that seen here, despite their transformation into rocks. Imaging techniques using x-ray computed tomography (also called "CT" or "cat" scanning) can reveal the inner structure of fossils-detailing their bony beginnings that distinguish them from ordinary rocksand any anatomical clues that can be linked to behavior or can help place the fossil into an evolutionary context. It is not clear what function these air cells serve in the skull's temporal bone but they are clearly different between humans and chimpanzees and will help diagnose whether fossils that preserve these air cells are more closely related to humans or to chimpanzees. Image courtesy of Cheryl Hill.

Figure 1.3 A high-resolution virtual slice through a human skull (left) and a chimpanzee skull (right) at the temporal bone near the ear. The spongelike bone is the inside of the mastoid process which is the palpable bump just behind the ear that attaches muscles from the collarbone to the head. These are not fossilized, but many times fossils will still contain much of the infrastructure of bone, like that seen here, despite their transformation into rocks. Imaging techniques using x-ray computed tomography (also called "CT" or "cat" scanning) can reveal the inner structure of fossils-detailing their bony beginnings that distinguish them from ordinary rocksand any anatomical clues that can be linked to behavior or can help place the fossil into an evolutionary context. It is not clear what function these air cells serve in the skull's temporal bone but they are clearly different between humans and chimpanzees and will help diagnose whether fossils that preserve these air cells are more closely related to humans or to chimpanzees. Image courtesy of Cheryl Hill.

tomorrow someone could dig up the LCA in Asia and turn everything upside down. That is the fundamental nature of paleoanthropology and of science.

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