Before the Dawn: Recovering the Lost History of Our Ancestors
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The figure shows the genealogy of an extended family—that of orangutans, gorillas, chimpanzees and humans. The tree was constructed by decoding part of the mitochondrial DNA of each species and comparing the common sequences of DNA letters, looking for differences. The lengths of the tree’s branches are proportional to the number of differences that have evolved between each species.
The bushiness of the chimpanzee branch reflects the amount of genetic diversity that has evolved in the chimp line. By comparison, the human branch is unnaturally straight, a sign of natural calamities that reduced the human population, or of modern humans having killed off all rival human species, or both. The Neanderthal sequence is derived from DNA extracted from fossil bones.
b Brain volume is a crude measure because it reflects many other things besides raw computing power, including body size and living in a cold climate. The Inuit, or Eskimos, have the largest brains of any modern humans, and Neanderthals had the largest brains of all. Paleoanthropologists often prefer to use a different measure, called the EQ, or encephalization quotient, which gives a measure of brain volume in relation to body volume. By this measure the numbers are as follows: chimpanzees 2, australopithecines 2.5, Homo habilis 3.1, Homo ergaster 3.3, modern humans 5.8. Source is ref. 9.
c Genes are strings of DNA that embody the information to make proteins, and proteins are the working parts of the living cell. But only some of the DNA in a gene contains the code for its protein; the rest is known as noncoding DNA. A mutation in the coding DNA usually interferes with the protein’s structure. But mutations in the noncoding DNA usually have no effect on protein structure and are called silent.
d A one-paragraph summary of human genetics: The human genome consists of 2.85 billion units of DNA in all, packaged in large, individual molecules known as chromosomes. A person inherits one set of 23 chromosomes from each parent, so that each cell of the body holds a total of 46 chromosomes. Before the eggs or sperm are generated, the number of chromosomes must be halved, since when egg and sperm unite it will double. But before the halving process, the germline cells make each chromosome inherited from the individual’s father line up with its counterpart chromosome inherited from the mother. Each pair of chromosomes then swaps corresponding chunks of DNA with its counterpart, so that a new pair emerges, each of which is now a medley of maternal and paternal genes. Each member of the new pair is tugged to opposite sides of the cell, which then divides to generate eggs or sperm. A special feature of this process concerns the 23rd pair of chromosomes, known as the X and Y sex chromosomes. Because the Y carries the male-determining gene, which must never be swapped into the X, the two chromosomes do not exchange genes, except at their very tips. Long ago, the Y chromosome was the same length as the X, but it has shed genes because, through lack of the diversity generated by the swapping process, many of its genes fell into disuse. Sperm carry either an X or a Y chromosome, whereas eggs always carry an X. Fertilization creates individuals with an X-Y pair (men) or an X-X pair (women). A consequence of this process is that people carry separately in their cells the set of chromosomes inherited from their mother and father; it is only when they come to generate their own eggs and sperm that the maternal and paternal genes are assorted into new, recombined chromosomes.
e Everyone carries about half of their father’s genes and half of their mother’s; so what happened to the half you didn’t inherit? It gets discarded, along with all the genes it contains. Each gene comes in a variety of different versions, known as alleles. By sheer chance, proportionately more of some alleles may get into the next generation, just by the luck of the draw, while fewer fall into the discard pile. The frequency of a given allele in the population may thus change considerably from one generation to the next, from 5%, say, to 33%, to 13%, to 55%, and so forth in a random fashion. But this random walk cannot go on forever. Sooner or later the frequency of the allele in the population will hit one of two numbers, 0% or 100%. At 0%, the allele is lost forever from the population. At 100% it becomes universal, i.e., the only version of that gene in the population since all other alleles are lost. When an allele becomes universal, geneticists say it is “fixed.” The time it takes for any allele to become fixed depends on the number of generations and the size of the population, being faster when the population is smaller. When an allele becomes fixed, the population is then set on a different path through evolutionary space than if the other alleles of that gene had remained available to it.
f The fingerprints of natural selection at work can be inferred by comparing silent mutations in DNA units (ones that don’t change the design of a protein) with significant mutations (ones whose presence causes a different amino acid unit to be specified in the protein).
g At a particular region of the prion protein’s gene, known as codon 129, the gene exists in two forms. Since a person has two copies of each gene, one from each parent, it’s possible to inherit 1) each of the two forms, one from each parent, or 2) two copies of the same form. Having two copies of the same form of the prion gene turns out to be a risk factor for mad cow and related diseases; having two different forms is protective.
h Greenberg died at the age of 85, the year after publishing the first volume of his work on Eurasiatic. Paul Newman, a linguist at Indiana University, recalls visiting him in his final illness. Greenberg mentioned his regret that he had not gotten around to classifying the languages of southeast Asia. “He looked at me,” Newman said, “almost with tears in his eyes, and said that without classifying them, he hadn’t finished his work with the world’s languages.” Another friend, Harold Fleming of Boston University, a specialist in African languages, paid a farewell visit together with Greenberg’s longtime colleague Merritt Ruhlen. The two were the last scholars to talk with him. The conversation turned to a recent work on Gilyak, an obscure language of northern Sakhalin. Fleming mentioned that there was a Gilyak word irf meaning fox or jackal. Greenberg was familiar with the word and noted that its “r” was trilled like a French “r.” “What a memory!” Fleming wrote in a tribute. “What a scholar! And what a shame that his vast and unique knowledge of human languages had to leave us, could not be electronically stored, and that such a great scientist had to die under a cloud of misguided criticism!” Source is note 287.
i The gene in question promotes good parental behavior in male prairie voles. Stuck in front of the gene is a section of DNA that changes length quite readily between generations. In a vole population the section exists in a spectrum of lengths. Males with the longer section look after their pups with devotion, males with the shorter sections are less attentive. In environments where good parenting pays off, males with the longer section will be more successful. But in environments where it does not, males with the shorter gene may predominate. The gene in question is called the vasopressin receptor gene. Humans possess the same gene and the variable section, but its effects in people remains to be understood. Source is note 346.