Carved in stone The fossils

As you can probably guess from the excitement with which paleontologists greet each new fossil find, hominid fossils are few and far between. This fact isn't especially surprising, given that great apes tend to have relatively low population densities (and our earliest ancestors probably did, too). That scientists have been able to find fossils of many early hominids at all is a cork-popping event.

The human species found in fossils

Paleontologists have found fossils for a large number of hominid species, including prehuman primates and human primates. In fact, various hominid species have been identified from fossil remains, as Table 16-1 shows.

Note: The A or H in the species' names is scientific shorthand. Instead of writing Australopithecus, for example, scientists simply write A. The term Australopithecus speaks to the origin of the fossil: southern Africa. H, of course, stands for Homo, which means wise. The name Homo sapiens means wise man.

Table 16-1

Hominid Species

Name

Years on Earth (Based on Fossil Finds)

A. anamensis

4.2 to 3.9 million years ago

A. afarensis (Lucy)

3.6 to 2.9 million years ago

A. africanus

3 to 2 million years ago

A. aethiopicus

2.7 to 2.3 million years ago

A. boisei

2.3 to 1.4 million years ago

A. robustus

1.8 to 1.5 million years ago

H. rudolphensis

2.4 to 1.8 million years ago

H. habilis

2.3 to 1.6 million years ago

H. ergaster

1.9 to 1.4 million years ago

H.erectus

1.9 to 0.3 million years ago (and possible 50,000 years ago)

H. heielbergensis

600,000 to 100,000 years ago

H. neanderthalensis

250,000 to 30,000 years ago

H. sapiens

100,000 years ago to today

As you can see, for most of the past 3 million or so years, multiple species of hominids have existed at any one time — a situation that persisted until about 25,000 years ago. In short, in the not-too-distant past, we shared Earth with human species other than our own.

The dates in Table 16-1 indicate the years from which fossil specimens of each species have been recovered. (For a review of how scientists determine the dates of fossils, see Chapter 2.) But — and this point is important — the lack of fossil evidence doesn't necessarily mean that a species wasn't around longer than the time periods indicated, only that scientists haven't found it. So even though the current dates indicate that a H. habilis, for example, lived from 2.3 to 1.6 million years ago, if H. habilis fossils dating from 1 million years ago are found, the dates would change. (It's also why you see the range for H. erectus and a note that this species also may have existed up to 50,000 years ago.)

The tricky task of separating one species from another

As Chapter 8 explains, a very useful way for determining whether two individuals are of the same or different species is to determine whether they can interbreed. If the answer is yes, the individuals are of the same species; a no answer means, they are different species. Obviously, this information isn't available for any of our fossil ancestors, so in this case, species names are simply a function of morphology, or body structure. All the fossils that look the same are assigned to the same species.

This arrangement sounds easy enough until you consider that the fossil record is sparse and fossil remains of hominids usually are extremely incomplete —part of the leg bone here, half a jaw there, part of the cranium somewhere else. Complicating matters even more is the expectation of finding both juvenile and adult individuals, as well as males and females which may vary in size. As a result, there's a fair bit of argument about whether a new find should be considered a new species or merely another representative of an existing species.

Using skull size and shape, jaw muscles, and limb length can help researchers distinguish between one hominid species and another, as follows:

1 Ratio of forelimb to hind-limb length: Humans' arms are proportionally much shorter than the arms of chimpanzees, and scientists find different fossil hominids with different arm-length ratios.

As bipedal locomotion (walking upright) developed, forelimb length shortened. (If you want to get persnickety, as scientists tend to do, you can say that forelimbs aren't really arms until the organism is walking on its hind legs — hence, the use of the term forelimbs.) Forelimb length is a good way to evaluate which group a fossil belongs to because it's relatively constant for individuals of different ages. The absolute lengths of forelimbs and hind-limbs change as the individual grows, but the ratio of the lengths is consistent over a range of individual sizes.

1 Skull shape: Humans have proportionally much larger brains than do the other apes.

This just in

Don't you just love it when science keeps discovering things? Every time anthropologists dig up a new skull, humans' view of the fossil record and what it says about human evolution can change. After a recent bit of digging, researchers now know that Homo habilis and Homo erectus lived at the same time.

This bit of info changes nothing in the chapter; it just reconfirms that in the past, more than one type of hominid was around at the same time. But it does provide a clearer picture of whether one hominid gave rise to another (as opposed to both having descended from a common ancestor). Some paleontologists thought that perhaps H. erectus evolved from H. habilis. The new fossil find shows that both species were around together, meaning that (1) they both evolved from something else or (2) H. erectus could still have evolved from an offshoot of H. habilis(you know — some habilisgot lost on the way to the office, ended up in a place with different selective pressures, and so on).

i The form and arrangement of teeth (dentition): Humans have markedly different dentition from other apes. We do a lot of our food processing with our hands rather than with our teeth, and we lack the powerful jaw muscles and large teeth that are characteristic of the rest of the primates (a group including the apes and other things like monkeys).

We humans have forelimbs, which just happen to be arms, but because we don't need them to reach to the ground when we walk, they can be shorter. Chimps can walk around on hind legs for a little bit, but soon resort to using their (longer) forelimbs again. Watch a chimp walk next time you're at the zoo; then try it with your comparatively short, little arms — you'll fall on your head! Why the difference? Because humans started to adopt a more upright posture that was selectively favored. The move to upright posture and the decrease in the importance of front limbs for locomotion would have happened at the same time.

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