Chapter 19.  Human Evolution

• Caution:  the points raised in this chapter and in lecture represent only a small fraction of  current research into human origins.  This is a complex and rapidly developing field, as is appropriate.  After all, we are our favorite species!
• Some interesting questions regarding human evolution:
• How are humans related to other primates?
• What is known about the human fossil record (from approximately 4.5MYA to the present)?
• How, when and where did modern Homo sapiens evolve?
• How did the traits which separate humans from non-human primates evolve?  These include exclusive bipedalism, large brains and intelligence, tools, language.
• Primate taxonomy:
• Prosimians (lemurs)
• New World monkeys
• Old World Monkeys
• Hominoids
• Apes
• Humans
• Some general primate characteristics relating to human origins and characteristics:
• Largely a tropical/subtropical group
• Mostly arboreal
• Grasping hands and feet
• Hindlimb dominant locomotion
• Well developed vision (poorly developed olfactory sense)
• Large brains
• Long life spans, small litters, extended parental care
• Note that many “human” traits are exaggerations of general primate characteristics.
• Chimpanzees, bonobos and humans share a common ancestor which lived about 5MYA.  See Figs. 19.2, 19.4, 19.6.
• Molecular (DNA) evidence can be used to deduce phylogeny and the dates of branching events (e.g. Figure 19.6).
• Coalescence times.
• Molecular clocks must be calibrated with fossil data.
• Gene trees vs. species trees – see Figure 19.5, 19.23.
• Fossil evidence: scanty but steadily improving.  However, when fossil evidence is scanty, each new find has the ability to substantially modify existing models, so stay tuned.  Here are some generalizations:
• All major evolutionary events took place in Africa
• Today there is only one human species, but this situation is unusual.
• There are two "human" genera, each with about a half-dozen species:
• Australopithecus
• Lived ca. 4.5MYA - 1.4 MYA
• Small brains, ca. 400-500 cc.
• Large, prognathic faces.
• Large teeth
• Species gradually replaced by species of Homo
• Homo
• Lived ca. 2.4 MYA to present.  Note chronological overlap with Australopithecus.
• Larger brains -- ca. 600-1600 cc. (Modern brains are ca. 1200 cc).
• Smaller, less prognathic faces.  This is a juvenile characteristic, and many human characteristics may have a neotenic origin (e.g. big heads with late closure of cranium, hair restricted to head/armpits/pubes, non prehensile toes, ventrally oriented vagina).
• Taller, longer legs.
• Less sexual dimorphism
• Species of the two genera coexisted chronologically, sometimes occupying the same habitats (like modern monkeys) or different habitats (like modern chimps and gorillas).
• Origin of modern Homo sapiens:  there is a wide range of theories, but these can be classified into two categories: multiregional evolution, single origin evolution ("Out of Africa").  See Fig. 19.17.
• Some dates (subject to revision as more fossils are found and dated):
• “Anatomically modern” humans appear in Africa ca. 100,000 (160,000?) YBP.
• “Anatomically modern” appear in Eurasia later, ca. 60,000 YBP
• Multiregional evolution
• Homo ergaster/erectus evolves in Africa, disperses to Eurasia.
• These populations of archaic humans evolve independently into anatomically modern Homo sapiens.
• Gene flow among Africa, Europe and Asia keeps the species together.
• Strongest evidence for this theory is the anatomical similarity between contemporary people in different geographical areas and archaic humans (H. erectus).
• On this theory, genetic differences between contemporary human populations are ancient.
• Single origin evolution (“Out of Africa” model).
• Homo ergaster/erectus evolves in Africa, disperses to Eurasia.
• Populations of Homo ergaster/erectus in Africa evolve into anatomically modern Homo sapiens.  These disperse to Eurasia, displace populations of archaic humans.
• On this theory, genetic differences between contemporary human populations are recent
• The models make predictions and are testable:

Prediction	Multiregional Evolution	Single Origin Evolution
Fossil appearance of modern humans	More or less synchronous in Old World	In Africa first
Transitional fossils (archaic to modern)	Should occur throughout Old World	Should occur in Africa only
Morphological continuity between archaic humans and modern humans in specific geographical regions	Yes	No
Genetic diversity	About equal throughout Old World	Highest in Africa, lower elsewhere, due to founder effect.
Time of divergence between human populations	ca. 1 million years ago	less than 200,000 years ago

• So which model is right?  Preponderance of evidence favors the Single Origin model.  The genetic evidence is especially convincing.  See Figure 19.25.
• Evolution of characteristically human traits (I am arbitrarily selecting two - bipedalism and intelligence).
• Bipedalism has many advantages and consequently numerous hypotheses to explain its evolution:
• Hunting (following migrating herds)
• Predator avoidance on the open plains
• It frees the hands
• An ecological explanation:  during Miocene, climatic change in Africa (cooler/drier) reduced resource density.  This increased the distance required to be traveled daily to satisfy group caloric requirements. Chimps responded by reducing group size, humans responded by becoming bipedal.  Key assumption of this hypothesis: bipedalism is energetically more efficient for walking than is quadripedalism. This is the case.)
• Intelligence.
• Fossil evidence: increases in cranium size, tools (Homo only or Australopithecus as well?, cave art, etc.).
• The brain is expensive:
• Consumes 18% of daily energy budget.
• Mothers bear the cost of belonging to a big-brained species, e.g.
• Painful, dangerous birth process.
• Extended maternal care of helpless young (humans are “secondarily altricial”).
• So, what benefits outweighed these costs?
• Traditional explanation: intelligence fosters tool making, hunting/gathering effectively in groups. (“Man the Tool Maker”, “Man the Hunter”.) This increases individual fitness.  Problem: non-human primates are obviously very bright, but do not seem to use their intelligence to these ends.  They use it to maintain social relationships.
• Social intelligence hypothesis: males and females form alliances and cooperate.  The individuals with the highest social skills have higher reproductive success.  (This can be observed in modern primates.) Positive feedback enhances social complexity and individual intelligence.
• A new relative (2003): Homo floresiensis from Southeast Asia: Slides.