Chapter 24. Human Origins and Evolution

Molecular analysis reveals that the human lineage split from the chimpanzee lineage about 5–7 million years ago.

Which great ape is most closely related to humans? That is, which is our sister group? Traditional approaches of reconstructing evolutionary history by comparing anatomical features failed to determine which of two candidates, gorillas or chimpanzees, is the sister group to humans. It was only with the introduction of molecular methods of assessing evolutionary relationships—through the comparison of DNA and amino acid sequences from the different species—that we had the answer. Our closest relative is the chimpanzee (Fig. 24.2), or, more accurately, the chimpanzees, plural, because there are two closely related chimpanzee species, the smaller of which is often called the bonobo.

Just how closely are humans and chimpanzees related? To answer this question, we need to know the timing of the evolutionary split that led to chimpanzees along one fork and to humans along the other. As we saw in Chapter 21, DNA sequence differences accumulate between isolated populations or species, and they do so at a more or less constant rate. As a result, the extent of sequence difference between two species is a good indication of the amount of time they have been separate, that is, the amount of time since their last common ancestor.

The first thorough comparison of DNA molecules between humans and chimpanzees was carried out before the advent of DNA sequencing methods by Mary-Claire King and Allan Wilson at the University of California at Berkeley (Fig. 24.3). One of their methods of measuring molecular differences between species relied on DNA–DNA hybridization (Chapter 12). Two complementary strands of DNA in a double helix can be separated by heating the sample. If the two strands are not perfectly complementary, as is the case if there is a base-pair mismatch (for example, a G paired with a T rather than with a C), less heat is required to separate the strands.

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HOW DO WE KNOW?

FIG. 24.3

How closely related are humans and chimpanzees?

BACKGROUND Phylogenetic analysis based on anatomical characteristics had established that chimpanzees are closely related to humans. Mary-Claire King and Allan Wilson used molecular techniques to determine how closely related the two species are.

HYPOTHESIS Despite marked anatomical and behavioral differences between the two species, the genetic distance between the two is small, implying a relatively recent common ancestor.

METHOD When two complementary strands of DNA are heated, the hydrogen bonds pairing the two helices are broken at around 95°C and the double helix denatures, or separates (Chapter 12). Two complementary strands with a few mismatches separate at a temperature slightly lower than 95°C because fewer hydrogen bonds are holding the helix together. More mismatches between the two sequences results in an even lower denaturation temperature. Using hybrid DNA double helices with one strand contributed by each species—humans and chimpanzees, in this case—and determining their denaturation temperature, King and Wilson could infer the genetic distance (the extent of genetic divergence) between the two species.

FIG. 24.3

RESULTS King and Wilson found that human–chimpanzee DNA molecules separated at a temperature approximately 1°C lower than the temperature at which human–human DNA molecules separate. This difference could be calibrated on the basis of studies of other species whose genetic distances were known from other methods. The DNA of humans differs from that of chimpanzees by about 1%.

CONCLUSION AND INTERPRETATION King and Wilson noted the discrepancy between the extent of genetic divergence (small) and the extent of anatomical and behavioral divergence (large) between humans and chimpanzees. They suggested that one way in which relatively little genetic change could produce extensive phenotypic change is through differences in gene regulation (Chapter 19). A small genetic change in a control region responsible for switching a gene on and off might have major consequences for the organism.

FOLLOW-UP WORK The sequences of the chimpanzee and human genomes allow us to compare the two sequences directly, and these data confirm King and Wilson’s observations.

SOURCE King, M.-C., and A. C. Wilson. 1975. “Evolution at Two Levels in Humans and Chimpanzees.” Science 188:107–116.

King and Wilson used this observation to examine the differences between a human strand and the corresponding chimpanzee strand. They inferred the extent of DNA sequence divergence from the melting temperature and made a striking discovery: Human and chimpanzee DNA differ in sequence by just 1%.

King and Wilson’s conclusions have proved robust as ever more powerful techniques have been applied to the comparison of human and chimpanzee DNA. Today, after sequencing entire human and chimpanzee genomes and literally counting the differences between the two sequences, we find that the figure of 1% still approximately holds. Genome sequencing studies give us superbly detailed information on the differences and similarities between the two genomes, revealing regions that are present in one species but not the other, and which parts of the genomes have evolved more rapidly than others.

Because the amount of sequence difference is correlated with the length of time the two species have been isolated, we can convert the divergence results into an estimate of the timing of the split between the human and chimpanzee lineages. That split occurred about 5–7 million years ago. All the extraordinary characteristics that set our species apart from the rest of the natural world—those attributes that are ours and ours alone—arose in just 5–7 million years.

Quick Check 1 Did humans evolve from chimpanzees? Explain.

Quick Check 1 Answer

No. Modern humans and modern chimpanzees share a common ancestor. Changes have occurred along both lineages: from the common ancestor to humans, and from the common ancestor to chimpanzees.