Human origins far from simple

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Understanding human origins is difficult for a number of reasons. Foremost, among them being the bewildering array of fossils that have been discovered and to which, it seems, specimens are continually being added.

Paleoanthropologists seek out, collect and study evidence of early humans.

They make a coherent picture of the relationships and developments of these beings. Until the mid-1940s, the field labored under the bias that humans were the culmination in a "Great Chain of Being."

Each new fossil discovery was seen as a "missing link" in a linear chain of human ancestry. New links were constantly being added, or, as some might say, "gaps" filled. The iconic depiction shows a modern human striding along, leading a line of progressively less-brutish-looking ancestors.

This notion failed as it became harder to force new finds into a successive line of descent. At some excavation sites it was evident the fossil specimens could not have all come from the same interbreeding population.

Surely, distinct groups had coexisted, but their physical makeup (morphology) suggested the groups were only distantly related. One couldn't be the direct descendant of the other.

The evidence demanded a more complex explanation. A genealogy that better fits the data has modern humans represented as a single twig on a very densely branching tree.

Relationships between all other forms of life are illustrated in this manner. There was no reason to treat the fundamental nature of human ancestry as different from that of other creatures. Objectively, the nature of human origins was no more unique in the history of life than was the nature of the origins of giraffes.

Very often newly discovered fossils prove not to have been from creatures in our direct line of descent. Rather they are from extinct side branches of the human "tree." They may be distant "cousins", but need not be distant "grandparents."

Surely, if one goes back far enough, the hereditary beginnings of modern humans and these "cousins" do join in the same population. It is only later that our lines of desin lap being decent diverged or took different paths.

Imagine tracing modern humans back on our particular twig on the family tree. Eventually, one arrives at a small branch. Other twigs arise from this common branch.

The branch grows from yet another branch and so on to the main trunk.

Each branch serves as a point of origin from which many other branches arise.

Biologists have names for the succeeding levels of branches on the tree of life. For our purposes, from branches designated as classes, of which we are mammals, spring orders. We are classified in the order of primates.

Primates include a family known as hominids. Hominids encompass a genus that includes orangutans, a genus that includes gorillas and another grouping that includes the two genera Homo and Pan.

We are Homo sapiens, meaning we are members of the Homo genus, but, within that genus, we are the species identified as sapiens. Chimpanzees and bonobos are members of the Pan genus, while also being classified in the broader, more inclusive tribe that also accounts for all of the Homo species.

These relationships are no longer dependent solely on comparative anatomy from fossils.

DNA analysis clearly demonstrates chimps and bonobos to be our closest living relatives. Though this tells only part of the story of our relationship, about 96 percent of our DNA is identical to that of chimps.

Except for Neanderthals, from whom some DNA has been recovered, such analysis is not possible for our more distant, extinct relatives. Modern humans and Neanderthals diverged from a common ancestral population 400,000 to 500,000 years ago. Neanderthals were on a developmental path that was a dead-end, and they became extinct.

The demise of Neanderthals was only the most recent dead-end for species within the genus Homo. There are many fossils of other species.

Each of these species has the unifying and defining characteristics of the genus Homo, but there are substantial variations.

In any population there is always some variation. Some variations become widespread and stable. If they are sufficiently distinctive these new characteristics become identified with a new species.

Such a population is said to have diverged from the old, to have started a new branch.

Its individuals have typically retained most of the characteristics of its ancestors while establishing some new. The divergent species physical appearance has become modified, but it is still recognizable as being related to the earlier species.

Among the family of hominids, only orangutans, gorillas, chimpanzees, bonobos and humans survive. Many of the extinct Homo species existed for a very long time. Some lasted for much longer than Homo sapiens have existed. Though most of our hominid kin are extinct, their fossils tell the story of our relationship.

Climate and environmental factors have played a significant role in the traits of those who survived. Modern humans do more to affect the environment than any other species that has ever existed.

In this way, we are truly unique. We are the only species capable of determining its own fate, as well as the fate of many other creatures.

Steve Luckstead is a medical physicist in the radiation oncology department at St. Mary Medical Center. He can be reached at steveluckstead@charter.net.

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