Limber 512 notes Nova "Children of Eve"
(while this is a somewhat old video, the issues are live and the conclusions and alternatives currently debated. See the several papers available on line from Scientific American. A recent examination of these methods can be seen in the recent 1/24/02 Nova video on Neanderthals.)
For a recent update, 5/05, see a recent NYT article. And very recently the entire DNA of a chimp, Clint, has been published. While much is unknown about the relation between DNA and behavior, nothing much has come along forcing scientists to revise the basics of evolution and even some of Darwin's own conclusions about human evolution.
An even more recent update, 1/16/07, in the magazine Science concludes:
"So what we infer is this: Modern humans spread out of Africa very late in the Pleistocene--as recently as 60,000 to 50,000 years ago. One founding population spread east, reaching Australia by 50,000 to 45,000 years ago. Another remained in southwestern Asia or India, but after ~5000 to 10,000 years, its descendant populations dramatically expanded their range, colonizing lands as far removed from one another as northern Africa, temperate Europe, and the Russian Plain. They also reached southern Siberia by 45,000 years ago (17) and arctic Siberia by 30,000 years ago (18), but the retelling of these and other events in the missing years of modern human evolution must await new fossil and archaeological discoveries as well as continued DNA sampling of the world's living populations. "
Science 12 January 2007:
Vol. 315. no. 5809, pp. 194 - 196
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The Missing Years for Modern Humans
The ideas of Wallace and Darwin involved random variation with advantages for a given environment preserved by survival and reproduction.
Variation results from sexual reproduction and more or less random mutations.
This occurs due to mate selection, advantages in feeding and other survival factors, as well as enhancement of offspring survival. For example, changes in climate are likely to offer differential opportunities for variants of a species to reproduce.
Of course chance plays a fundamental role, as do developmental factors. Finally it is a BIG mistake to think that the "survival of the fittest" implies there are intrinsic advantages to size or strength or any characteristic indendent of a particular set of circumstances. Smaller, faster reproducing monkeys are increasing their populations in some areas while bigger," smarter" chimps and orangs face extinction.
They observed the following facts:
1. the potential for exponential population growth
2. in fact, populations are generally stable
3. there are clear limitations in resources
4. individuals are unique
5. these unique differences are heritable
They inferred the following:
1. there was a "struggle" for existence among individuals
2. there was differential "survival of the fittest" individuals (natural selection)
3. species evolved over generations as similar individuals survived and perpetuated their useful characteristics (for that "niche").
(Consider Darwin's finch example.) From one pair of finches, at least 13 species have evolved with varying distinctive beaks adapted for particular niches (e.g. food types).
Recall the title of Wallace's classic paper--Wallace, A. R. (1858). On the tendency of varieties to depart indefinitely from the original type. In Evolution by Natural Selection Cambridge: Cambridge University Press.
"gradual" change does not necessarily imply a long period of continuous changes. Instead changes may occur rapidly, though small in each successive generation--as in a period of dramatic or periodic climate change.
(The video narrator underestimates the variety of evidence Darwin used; hominid fossils played only a minor role. See "Descent of Man" readings ; also Darwin's "tree sketch." from his notebooks.)
This compares very well with current views of ape-human relationships. Of course Darwin and others of the 19th century were way, way off in the timing of evolution. The earth is much older than anyone then conceived; this posed a problem for Darwin since there was little time for a gradual process of repetitive variation and selection to derive the enourmous variety of species known to exist. Here is a sketch approximating beliefs about time and relationships today among apes and humans:
This is the most modern means of assessing similarity although Darwin's supposition that shared diseases between species indicates a close relationship is a nice 19th century approximation.
(see xxx for more information.)
This was an early method.
We share 97% gorilla, 98% chimp genes. This is a very crude index however since we don't know much about what sections of DNA do what--if anything. Recently (1997) it has been shown we share over 40% of our DNA with yeast! This makes percentage comparisions somewhat worthless for predicting morphology and behavior just from DNA at present (2005) Several 2007 papers in "News" begin to deal with just what it means to share DNA. And even the idea of "gene" is a moving target.
( "Genetic" circa 1900 meant origins or growth. The word "gene" itself may have been first used in 1909 based on an earlier term "pangen" (pangenesis) designed to capture Mendel and others notion of the smallest bit of inheritable features. Darwin had his own --very bogus-- version of pangenesis in which the germ cells (egg and sperms) gathered information from all parts of the body and somehow conjoined that information at mating. This model was designed to explain Larmarckism as a component of reproduction-- which we now know is totally false.)
See Sankaran, Neeraja
The Century of the Gene (review)
Journal of the History of Medicine and Allied Sciences - Volume 57, Number 1, January 2002, pp. 106-108)
Mitochondrial DNA (mtDNA) transmitted exclusively through maternal genes provides another useful index of similarity.
This is one of the serious problems in terms of the precision of these "molecular clocks."
Entire theories can be overturned with the discovery of a new fossil. One must ask how representative the fossil is of its population. This requires assumptions about age, sex, degree of sexual dimorphism, and of course range of variation. Currently, most of the fossil remains come from Africa; yet one must wonder if Asian regions have had comparable investigation. One reliably dated million year old or less humanoid fossil from China, for example, can change our entire perspective.
Fossils, too, must be dated accurately.
(Since this video, there has been some suggestion that Lucy was not a female! This doesn't lessens the value of the find but does highlight some of the difficulties in using fossil evidence; in addition to dating one must estimate age and sex to project an estimate of the population characteristics.)
1 humans are wide at thigh, close knee vs parallel legs of chimp. Lucy was "knockneed" with a chimp-like pelvis. it is likely she was tree-dwelling creature? Advantages of bipedalism include efficient land locomotion, freeing of hands for carrying, weapon use, etc. At some point after Lucy's era, human ancestors were able to run with great endurance as modern leg muscles and tendons evolved.
2 human curved pelvis (Pelvic changes were forced by bipedalism and perhaps brain size- larger brains require a larger cervical opening.)
1 first tool users
How similiar to homo S? Ancestor or distant cousin? They certainly had cultural and complex rituals far beyond any existing critter. Hunter-gatherers must have a high level of intellectual and social development . Size of fossil remains indicates great size, strength, and endurance. Beyond physical dimensions, most claims are shear speculation. Some recent DNA evidence recovered from bones supports the idea modern humans are not decended from Neanderthals. (Maybe more on Neanderthals later--a recent theory is that they were clumsy as indexed by their relatively small semi-circular canals.)
Early modern humans differed from Neanderthals in reduced size, endurance, and strength. Probably these were more intellectually efficient and did not require the size etc.
1 the problem of local variants --why do people from various ethnic groups look similar-- as well as have certain size and nutritional variation?
2 new migrations or interbreeding?
Using information about the patterns and roles of specific genetic material, investigators can show that widely differing species share a common early developmental history.
(See Science, 1997 for a recent discussion)
The use of DNA changes requires assumptions on constancy of rate of change.
A recent < <1 million years ago migration that superceded many previous emigrations from Africa. What happened to those people? How come people look so different? Could these changes take place within a million years? It is close and depends on fine-grained dating of existing fossils. The video shows mitochondrial DNA (mtDNA) similarity methods.
1 multiple origins in parallel?
2 interbreeding of recent emigrants, essentially retrofiting the brains of existing inhabitants yet keeping their bodies with local variations of size and shape, etc.?
3. ???????? (you name it!)
4. There are raging controversies about these matters ongoing today involving questions about genetics, biochemistry, and racial differences. At present it seems while there may be slight statistical differences among the various human "races", the differences are slight and not much more than differences between individuals from the same populations. Phenotypic features such as facial and head structure and pigmentation--while perceptually salient--are only very slight genetic differences ESPECIALLY when compared with genetic differences between members of other primate species--which have greater DNA variation.
Recent findings support the idea that evolution has continued into recent years in the realm of diet and resistence to disease.