Wallace's paper, "On the tendency of varieties to depart indefinitely from the original type", concludes: "We believe we have now shown that there is a tendency in nature to the continued progression of certain classes of _varieties_ further and further from the original type -- a progression to which there appears no reason to assign any definite limits -- and that the same principle which produces this result in a state of nature will also explain why domestic varieties have a tendency to revert to the original type. This progression, by minute steps, in various directions, but always checked and balanced by the necessary conditions, subject to which alone existence can be preserved, may, it is believed, be followed out so as to agree with all the phenomena presented by organized beings, their extinction and succession in past ages, and all the extraordinary modifications of form, instinct, and habits which they exhibit."
Darwin thought it possible that all life evolved from a single cell organism.
Darwin and Wallace's (1858) theory had four steps:
Organisms differ from each other in features that are inherited
More are born than can survive. (Fitness is determined by survival rates -NOT strength, size, etc.)
Certain of the inherited features increase the probability of that individual's offspring surviving
this selection leads to the accumulation of favored variants which over long periods, aided by isolation, dramatic climate changes, etc. create "new" species.
this is often guesswork including a large indeterminate element of chance,
"just so" stories are part of understanding evolution. See what Darwin said
about the functions of language:
"As the voice was used more and more, the vocal organs would have been strengthened and perfected through the principle of the inherited effects of use...but the relation between the continued use of language and development of the brain, has no doubt been far more important....we may confidently believe that the continued use and advancement of this power would have reacted on the mind itself, by enabling it and encouraging it to carry on long trains of thought." (Origins, 1871)
Note both the good and bad ideas in hindsight -- including a Lamarckian twist common until the early 1900s.
Offspring inherit features from parents but are not identical to them. These variants offer a source of adaptations and also species resistance to infectious organisms. (Also the duplicate genes offer some protection against defective genes -lost for example in sex-linked genes as in color blindness.)
e.g. competition among males, female preferences
Both M and F are obviously necessary but mammal mothers "invest" far more than males who may only provide sperm in some species but protection, food, and parental care in others.
It is a useful fiction to think of mammals as built up out of many parts, like a pattern composed of small tiles. Evolution can selectively enhance or reduce any of those individual tiles if overall fitness is increased. (In reality, individual genes may have many diverse and seemingly unrelated effects - e.g. the Williams syndrome.)
One very important way species evolved from a common ancestor, is by speeding up or slowing down biodevelopmental processes. Primates and humans in particular have retained fetal and infant characteristics of mammals into adulthood. Our large brain, some bipedal features -even the geometry of female genitals enabling face to face intercourse may be a consequence of neotenous processes.
Such historical changes select for different adaptive features to better cope with new circumstances. Our hominid ancestors evolved over the last 7 million years in response to such changes.
three times the size of chimpanzee brains
large prefrontal cortex, enlarged cerebellum and hippocampus relative to an average primate of our size
The neotenous extension of brain development exposes our human brain to environmental -cultural influences during its formation hence shaping it's growth. Coupled with prematurity and extensive parental care, the human brain is much more a product of its extrauterine environment than others. Neural "pruning" of surplus neurons and synapses, as well as formation of use-driven synapses builds "neural circuits" during these early years.
enables speech, throwing and other tool use.
The above distinctly human traits require increasing support from fathers for food and protection. This in turn probably required closer sexual and interpersonal relationships, including some assurances of fathering the child he is supporting.
Female apes (chimps, orangs, gorillas) generally have interest in sex only when they are ovulating - - with some exceptions, e.g. bonobos. They also typically have a strong sensory display - visual swelling, pheromones - that signal that receptivity. And of course the males respond immediately to those signals. Humans are much more sexually active around the clock and have more subtle sexual signals that develop stronger interpersonal relationships necessary to maintain MPI.
All of the above make it possible for females to become fertile within a year or two - hence enabling human females to more than double the reproductive rate of their ape cousins. (Continued nursing by a clinging infant or juvenile prevents ape mothers from going into estrous.)
Culture is a kind of external memory of symbols and artifacts. It enables an accumulation of knowledge from generation to generation without reliance on DNA other than to access it - as in the case of human language. It is likely that ability to use culture shaped our ancestor's brains via natural selection; see the "Baldwin effect" below.
Humans are adapted for a high-energy diet.
Humans have a unique synergy of brain, language, and culture. Differential brain structures enhance planning, memory, analyzing patterns, predicting social interactions and communication.
Several of the above can be seen from a social perspective, e.g. language, MPI, theory of mind.
Human evolution was not simply a biological process; selection factors included ability to utilize existing artifacts, including language, fire, weapons and tools, as well as engaging in successful social interactions. Thus our brains and bodies can be viewed as cultural adaptations as well as biological ones.
C. (1871/1981). The descent of Man, and selection in relation to sex
(Photoreproduction of the 1871 edition ed.). Princeton: Princeton University
Gould, S. J. (1977). Ontogeny and phylogeny . Cambridge: Harvard University Press
Jones, S., Martin, R. D., & Pilbeam, D. (Ed.). (1992). The Cambridge encyclopedia of human evolution. Cambridge: Cambridge University Press
Limber Psychology of Primates webpage (http://pubpages.unh.edu/~jel)
Wallace, A. R. (1858) On the tendency of varieties to depart indefinitely from the original type.