Bushes And Trees

Until fossils come with tags that describe exactly what species they belonged to, what they ate, who they mated with, and whether or not they could speak, there will always be arguments over their interpretation. The issue is not whether fossil hominins hold clues to our evolution, the issue is how to interpret those clues and come to a logical consensus on how Homo sapiens came to be.

The first obstacle in coming to such a consensus is that hominin fossils are rare. Sure, it is hard to visit any part of the world without running into humans today, but our insect-like colonization of Earth is a very recent phenomenon. Prior to 10 Kya all humans lived in small groups rarely exceeding 150 individuals. So the chances of finding a hominin fossil are diminished purely because of past population sizes. Paleoanthropologists find many more bones and teeth of rodents and bovids (antelopes and the like) than they find hominins, but there are still thousands of hominin fossils on record.

Because of two fundamentally different views of interpreting evolution, there are arguments over which fossils should be included in the lineage leading to humans and which should be placed on side branches. Basically, the "splitters" see the hominin phylogeny as a bush, but the "lumpers" see it as a tree. The most extreme lumpers see it as a saguaro cactus, with maybe two or three lineages/branches at most.

The bushy view provides a very complicated phylogenetic history of hominins with multiple species existing at any one time. The tree or cactus symbolizes the view that hominins have evolved with very little diversity and with very little overlap in species, in long evolutionary lineages.

The splitters argue that the fossil record cannot possibly contain representatives from all of the hominin species that ever existed; that there must be so much diversity that is not being sampled because of preservation issues. The lumpers' view is a conservative one that points to evidence from the fossil record as we know it.

The bush-versus-tree argument is important if we are to determine the nature of human evolution. It is also important for how we reconstruct the daily lives of hominins 1.5 million years ago. Life may be a whole lot different if there was a separate species of 3-foot-tall bipeds sharing the planet with us, and that was what life was like for humanlike H. erectus which coexisted with ape-like robust australopiths and for Indonesian humans about 13,000 years ago when the so-called "hobbits" lived on

Flores. What if being the only bipedal tool-using ape, like we are now, is an exception rather than the rule? --

Why "Hominin"?

"Hominin" is replacing the once ubiquitous term "hominid" in the literature. Both terms are used for humans and their extinct ancestors since the last common ancestor with chimpanzees (LCA) and both continue to be accepted. The difference in the terms has to do with how scientists view the relationship between humans and the rest of the great apes. Traditionally humans were the only species to belong in the family Hominidae ("hominids"), with the great apes placed in a separate family, Pongidae, because anatomical similarities group the great apes to the exclusion of humans. However, due to the genetic similarity between chimpanzees, humans, and gorillas (to the exclusion of orangutans), chimpanzees, gorillas, and all their fossil ancestors are increasingly lumped in with humans in Hominidae. Therefore subcategories were created to further differentiate the groups, where humans and their fossil ancestors are called hominins.

THE LAST COMMON ANCESTOR

What would the last common ancestor look like? What would the earliest hominin look like? How much would it resemble a modern chimpanzee? Paleoanthropologists must show that their fossils are not fossil chimpanzees or gorillas in order for them to be accepted as part of the hominin family tree. This is a tall order for the early part of the hominin fossil record (between 7 and 4 Mya) when hominins were still very ape-like. Plus there are no fossil chimpanzees or gorillas from the late Miocene to the Pliocene to hold up for comparison.

There is only one recognized fossil chimpanzee on record and it dates to the relatively recent mid-Pleistocene. A few teeth were collected from a site near Lake Baringo, Kenya. Argon-Argon dating put the teeth at about 525 Kya. They look exactly like modern chimpanzee teeth and, interestingly, there are fossil humans from the same regional localities, which presumably lived in the same place in prehistory as the chimpanzees.

The tropical habitats of chimpanzees and gorillas are to blame for their near absence from the fossil record. Modern African apes stick to warm wet tropical forests and are assumed to have enjoyed the same habitats in the past. Fossilization rarely occurs anywhere, let alone in these kinds of habitats where soil acidity is high and where organisms that devour carcasses are thorough and efficient.

It is difficult to recognize and properly identify primitive species near major evolutionary divergence events, like the LCA and its relatives. On top of that, how do we gauge within and between species variation for extinct creatures? Basing variation estimates on what we see in monkeys, apes, and humans today is the most logical and reasonable way to overcome this problem, but it also risks masking different patterns of variation that may have existed in the past.

There are other obstacles in paleontology. Often the fossils are broken or fragmented. Some of the most diagnostic anatomy in the skull is also the most fragile. Knowledge from years of anatomical study and experience is required to piece together fragmentary bones but with the help of modern imaging techniques, like CT scanning, the process is becoming much easier. With virtual fossil reconstructions, paleontologists are able to repair fossil breaks without having to clean, extract, and assemble them by hand which often damages fossils even further.

The number of juvenile hominin fossils that are discovered can also lead to issues in their identification. Individuals that are not fully adult or fully grown do not have all of the diagnostic features of their species. A good proportion of important hominin specimens are infants or children, for instance, the Taung Child (Australopithecus africanus), the Nar-iokotome H. erectus boy, the Dikika baby (Australopithecus afarensis), the Mojokerto Child (Homo erectus), and there are several juvenile Neanderthals. Their ages are determined by their stage of dental eruption. Both the deciduous or milk teeth and the permanent second set of teeth erupt in a particular pattern and follow a particular schedule. For instance, the human first molar erupts around six years of age, and so forth. There is a similar pattern and schedule to the fusion of growth plates at the ends of the long bones. Differences in body size and muscle attachment size on the bones indicate whether or not the individual was male or female, but such differences are much less pronounced before adulthood so determining the sex of juveniles is difficult the younger they are.

Since fossils are relics of a creature's structure, species are identified by anatomical traits—mainly highly diagnostic features of the skull and teeth. For many of the terms used in the following anatomical discussions, please refer to the figures and to Appendix A and for the context of the species within the hominin phylogeny please refer to Figure 1.5.

Table 3.1 Trends through Time in the Plio-Pleistocene Hominin Fossil Record

• Reduction of face size

• Reduction in prognathism, or the projection of the face

• Reduction in molar size, relative and absolute

• Reduction in canine size

• Loss of arboreal characteristics like long curved fingers and toes, short legs, and long forearms

• Acquisition of terrestrial and bipedal characteristics like shorter toes, longer legs (especially the femur), and shorter arms

• Increase in cranial capacity (which is used to infer brain size)

• Increase in skull roundness

• Increase then decrease in browridges

• Increase in stature and body size

• Decrease in the robusticity of bones

• Decrease in sexual dimorphism, or the differences between males and females in body size and tooth and skull characteristics

In general there are trends in the hominin fossil record we can follow to track our ancestors' journey from LCA to us (Table 3.1).

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