Reconstructing the history of hominid evolution

With all these hominid species, scientists are still trying to figure out which species may have given rise to which other species along the lineage that led to humans. Because no one can say with certainty that a fossil represents a common ancestor of two other species (the fossil may represent a closely related dead end, for example), this task is fairly challenging.

Figure 16-2 shows a hypothetical evolutionary pathway from an ancestral species similar to the extant apes and leading to modern humans. It includes a sequence of intermediate species with increasingly large brains, reduced jaw musculature and dentition, and the evolution of bipedalism (walking on just two feet). I've thrown in stone tools and fire just for fun.

The actual intermediate species may not be the ones scientists have already found, or they may be; knowing for sure is impossible. What scientists do know is that we can reconstruct the evolutionary events that led from an apelike ancestor to a modern human through the series of fossil species that have already been found. The following sections describe some of the major players; consider this a sort of hits list of the hominid fossil record.

Lucy in the sky with diamonds: Australopithecus afarensis

This famous fossil, called Australopithecus afarensis (A. afarensis, for short) is commonly known as Lucy. Named for Lucy in the Beatles song "Lucy in the Sky with Diamonds," which was playing at the time, Lucy was such an important find because a large part of her skeleton was found together, which gave paleontologists a fair bit of confidence in describing the species.

Lucy was found in 1974, but her species lived from about 2.9 million to 3.6 million years ago. She was about 3>2 feet tall and weighed 60 pounds. She was bipedal. At the time of her discovery, Lucy was the earliest bipedal hominid that had been discovered. The relative length of her forelimbs is intermediate between that of apes and people.

Figure 16-2:

hypothetical evolutionary pathway leading to modern humans.

Figure 16-2:

hypothetical evolutionary pathway leading to modern humans.

Evolution Des Hominid

Before Lucy's discovery, many researchers believed that the driving force behind the evolution of the large brain in the human lineage was that once the hands were free (due to the development of bipedalism) to use the tools, smarter individuals (those who were better toolmakers) would have an advantage. This idea is interesting, but it turns out that Lucy's brain was no bigger than a chimpanzee's — about one third the size of a human brain.

4jtJABCi What this fossil species (and others since) have made clear is that the evolution of bipedal locomotion occurred before the evolution of a large brain. A long part of the human family tree is populated with ancient hominids that walked upright but had small brains. This is a nice example of a case in which the evidence provided by a fossil find allows scientists to reject one potential hypothesis about the pathway of human evolution. So it's back to the drawing board to come up with hypotheses that explain why a big brain was all of a sudden favored by natural selection.

The fossil record shows that after A. afarensis, the hominid lineage split into two branches. One branch eventually led to humans; the other branch led to a group referred to as the robust Australopithecines: A. boisei and A. robustus. These species had very strong jaws (perhaps for eating plant material). This lineage, which also includes A. aethiopicus, persisted from about 2.7 million to 1 million years ago and then became extinct.

I'm a traveling man: Homo erectus

Another important branching event in the hominid family tree is the one that separated H. erectus from the lineage leading to H. sapiens. H. erectus originated around 1.9 million years ago and went extinct almost everywhere 300,000 years ago, though one subspecies may have persisted on the island of Java perhaps as recently as 50,000 years ago.

H. erectus had a substantially larger brain than the Australopithecines, made and used tools, and may have been able to control fire. But the species' greatest claim to fame was being the first hominid species to leave Africa, which it did around 1.5 million years ago. By 1.2 million years ago, it had reached China and Southeast Asia.

H. erectus had a brain about two thirds the size of the human brain, but analysis of the internal structure of the H. erectus skull suggests that the area of the brain involved with speech wasn't developed to the extent that it is in later hominids. Meaning? Language came later.

Homo sapiens and Homo neanderthalensis

The last two species on the hominid tree are Homo sapiens and Homo neanderthalensis. Both species originated in Africa — H. neanderthalensis about 250,000 years ago and H. sapiens about 100,000 years ago — and both moved out of Africa. H. neanderthalensis colonized Europe and parts of Central Asia, whereas H. sapiens went on to colonize the whole world. H. neanderthalensis coexisted with H. sapiens for a long time and went extinct only about 30,000 years ago, but anthropologists don't know why.

H. neanderthalensis was more robustly built than H. sapiens, possibly as an adaptation to the cold, but the two species' brains were the same size. Evidence has been found that H. neanderthalensis had advanced cultures: They modified their environment for shelter; they had art; and they buried their dead (the reason why a good fossil record for the species exists).

Many drawings of H. neanderthalensis incorrectly portray the species as having a hunched posture. The reason for the mistake? An early specimen had extreme arthritis. Analysis of many additional specimens reveals an upright posture like that of humans. If you were to pass a specimen of H. neanderthalensis on the sidewalk, you might notice the stockier build and facial features (such as a more pronounced brow and perhaps a larger nose), but if he was wearing a nicely tailored suit, you might not give him more than a second glance.

Hobbit Man: Homo floresienses

An incredible and still somewhat controversial fossil find, Homo floresienses, discovered on the Indonesian island of Flores in 2004, was possibly a dwarf species of the genus Homo. Nicknamed "Hobbit Man" after the character in J. R. R. Tolkien's The Lord of the Rings, the first skeleton found was a female approximately 3 feet tall, weighing perhaps 50 pounds, and estimated to be approximately 30 years old at the time of her death. The fossil evidence suggests that the species inhabited the island as recently as 13,000 years ago. (Homo Neanderthalensis went extinct approximately 25,000 years ago.) Because of the limited fossil record (so far, only one fossil has been found with an intact skull), no one knows exactly where H. floresienses fits in, but here are some suggestions:

1 It's a dwarf form of Homo erectus.

1 It's a Homo sapiens afflicted with microcephaly, a condition characterized by an unusually small head and mental impairments (This idea gained traction based on the fact that the H. floresienses'brain case is so small).

1 It's a new species that lived into modern times — an interesting conjecture that's not supported by any physical fossil evidence but that has traction because people native to the island have legends of small furry people who lived in caves and had a different language. Sightings of such creatures were mentioned as recently as the 1800s and continue to this day on the island of Sumatra.

Did H. sapiens and H. neanderthalensis interbreed?

The fossil record seems to indicate that Homo sapiens and Homo neanderthalensis had far more similarities than differences. That being the case, was H. neanderthalensis truly a separate species? Maybe, as some researchers have suggested, it was simply another subspecies of H. sapiens, and possibly — just possibly — during the many years that the species overlapped in range, they interbred. This possibility is interesting, but you can't find clues in the fossil record. Instead, you have to turn to DNA.

Luckily, DNA is tough stuff, and advanced retrieval techniques have made it possible to obtain sequences of nuclear DNA from H. neanderthalensis specimens almost 40,000 years old.

The human and neanderthal DNA sequences are very similar, but there's about one half of a percent (0.5) difference. This is enough to identify specific DNA sequences that are specific to one or the other species.

Here's what DNA testing has revealed: The H. neanderthalensis genome is clearly distinct. Had any significant amount of mixing occurred, this result would be less clear. So DNA evidence clearly weighs in against the idea that the two groups interbred. Scientists can't rule out the possibility of any mating between the two species, but they can rule out the possibility that any significant genetic mixing occurred.

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