Gadget by The Blog Doctor.

Thursday, October 28, 2010

Human Origins


As the name indicates, the video below is a quick introduction to Human Origins.

The video is one of about 25 that can be found on this web page (about half way down).

Humans and the Great Apes

Some confused people want to claim that according to evolutionary theory Humans evolved from Chimpanzees. This is not the view of scientists. The diagram below shows the general view of the relationship between the human line and the chimpanzee line. About 6 million years ago there lived a creature part of whose population started to evolve in the direction of chimpanzees and another part of whose pouplation began to evolve in the direction of humans. That creature is the Last Common Ancestor (LCA) of Humans and Chimps. The amount of time ago that the creature lived is estimated by the molecular clock. See also this article, which indicates that 6 million years ago is very likely an upper limit to the time of the LCA, which might well be a million years or so closer to the present.

A major problem is that scientists have found few fossils along the branch leading to Chimpanzees. The hominid line leading to us, on the other hand, currently has about 19 species populating it, as shown by the diagram below. (Click on the diagram for an enlargement, in a new window or tab.)

Another way of graphically viewing the hominid fossils is via this interactive timeline, which contains links to articles on the different species.

The Hominids

I won't discuss all of the 20 or so hominid species as that would lead to a very long and tedious post. This link has summaries of the hominids (and it is still fairly long). At the right there are links to the hominid species (mostly links to Wikipedia articles).

Sahelanthropus tchadensis

Here is a link to a photograph of the fossil cranium.
Fossils of this species date probably to almost 7 million years ago. There is considerable debate about this creature.    Some experts, including its discoverers, think that it has remarkably modern features, such as its flat face and indications that it was bipedal. The problem with this is that Sahelanthropus' age makes it comtemporaneous or possibly older than the LCA. It might be that the LCA was much more human like that previously expected.

On the other hand, there is some uncertainty about the stratiography of the find, and the fossil might be less than 7 million years old. As well some experts are unconvinced about its modern features - see this link for an such an opinion.

More fossil evidence will be required to resolve these issues. The discoverers of Sahelanthropus have many more fossils, some of which might help to clarify the status of the creature, which might include clarification of the nature of the LCA.

Here is the website of the discoverers, giving their opinion of the fossil.

Orrorin tugenensis

Here is a link to a graphic of the Orrin fossils, which are between 6.2 and 5.6 million years old.

The discoverers claim that Orrorin was bipedal, and there are indications that this was true, but that issue has not been tied down conclusively. The creature was about the size of a modern Chimpanzee.

Links to articles on Hominids

   Sahelanthropus tchadensis

   Orrorin tugenensis

   ArdiPithecus kadaba

   Ardipithecus ramidus

    Australopithecus anamensis

   Australopithecus afarensis

    Kenyanthropus platyops

   Australopithecus africanus

   Australopithecus garhi

   Australopithecus sediba

   Australopithecus aethiopicus

   Australopithecus robustus

    Australopithecus boisei

    Homo habilis

    Homo georgicus

    Homo erectus

    Homo antecessor

    Homo heidelbergensis

    Homo neanderthalensis

    Homo floresiensis


Ardipithecus ramidus

A few Ardipithecus ramidus fossils were found in 1990 but the motherload as pictured at right was found in 1994. The fossil was very fragile and took a great deal of expertise, care and time to extract. It also took a great deal of time to preserve, reconstruct and analyse it, and it was not until 2009 that comprehensve papers were published on Ardi. The papers covering the fossil evidence as well as environment that Ardi inhabited and its place in human history can be downloaded from this link. Ardi is about 4.4 million years old.

The reconstructed scull is similar to Sahelanthropus tchadensis, and is about the size of a Chimpanzee. The useful summary below is found at this site:

Ar. ramidus was quadrupedal in the trees, walking along branches using the palms of their hands, as many monkeys do, (i.e. they weren't hanging from branches or climbing with a vertical torso like modern apes do). It is also claimed by its discoverers that ramidus was bipedal on the ground, though not nearly as well adapted to it as humans are. (This particular claim is being treated cautiously so far; a few important scientists have expressed reservations about it.) The foot of ramidus had a widely divergent big toe for grasping while climbing (like chimps), but lacks the extreme feet flexibility of chimps which allows them to mold their feet around objects. ramidus also has none of the anatomical specializations of chimps for knucklewalking when on the ground.

The Ar. ramidus pelvis was very ape-like in the lower pelvis, but had changes in the upper pelvis which made it an effective upright walker. It is more primitive than the pelvis of australopithecines, which were almost fully adapted to bipedality.


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