Chapter 32   Human Evolution

Primate images from Wikimedia Commons

32.1 Evolution of Primates

A.  Primate Characteristics

1.     Primates differ from other mammals by being adapted for arboreal life (living in trees).
2.     Forelimbs and Hindlimbs

a. hands and feet have five digits each.
b. flat nails replace claws and sensitive pads develop on the underside of fingers and toes.
c. Many primate hands have an opposable thumb; some have an opposable big toe.
d. These features allow the free grasping of tree limbs

3.     Binocular Vision

a. Primates have a reduced snout and the face is relatively flat.
b. The sense of smell is generally reduced.
c. The eyes are moved to the front of the face provide stereoscopic vision.
d. Cone cells provide greater visual acuity and color vision but require bright light.

4.     Large, Complex Brain

5.     Reduced Reproductive Rate

a. Primates have more single births
b. The period of parental care is extended with an emphasis on learned behavior and complex social interactions.

B.    Sequence of Primate Evolution

1. All primates at one time shared one common ancestor; prosimians were an early group to diverge and African apes were the last group to diverge from our lineage.
2. Prosimians diverged first and are most closely related to the original primate.
3. Anthropoids

a. Surviving anthropoids are classified into three superfamilies:
New World monkeys, Old World monkeys and hominoids (apes and humans).
b. New World monkeys reside in South America and Old World monkeys evolved in Africa.
c. New World monkeys (e.g., spider monkey and capuchin) have long prehensile tails and flat noses.
d. Old World Monkeys (e.g., baboon and rhesus monkey) lack prehensile tails and have protruding noses.

4.     Hominoid Evolution

a. About 15 MYA, dozens of hominoid species arose.

b. Proconsul was one species that lived at this time, and it is believed to be the ancestral ape.

32.2  Evolution of Early Hominids

1. The designation hominid includes humans and several extinct species related to humans.
2.  Fossil and anatomical data indicate ancestors of African apes and the human lineage diverged about 7 MYA.
3.  When such changes accumulate at a constant rate, it constitutes a molecular clock to indicate relatedness; these data indicate we diverged about 6 MYA.

A.    Comparing Humans to Chimpanzees

Compared to chimpanzees, in humans 

• The skull is in the midline of the body.
• The longer curved spine places the center of gravity over the feet.
• The broader pelvis and hip joints prevent swaying when walking.
• A longer neck on the femur in humans causes the femur to angle inward at the knees.
• The human knee joint is modified to support the body’s weight.
• The human toe is not opposable but the foot has an arch for long distance walking.

B.    The Early Hominids

• Until recently, science thought that the climate changed forests into savannas; there is little evidence of a shift in vegetation at 6 MYA.
• Additional advantages of bipedalism include reduction of heat stroke and carrying food back to females.
• While still living in trees, the first hominids may have walked upright on two feet (bipedalism) to collect overhead fruit.

Watch "Facts about Human Evolution" by SciShow, 12 minutes

Discuss:

1. Why did the "multiregional hypothesis" fall out of favor?
2. Describe the "Out of Africa" hypothesis.
3. What is the relationship between cooking and human evolution?
4. Why was it important that a social structure exist for early hominids?
5. How do we know that neandertals lived at the same time as homo sapiens?

32.3 Evolution of Later Hominids

• Australopithecines (genus Australopithecus) evolved in Africa 4 MYA.
• Australopithecus africanus, date about 2.8 MYA.
• Australopithecus robustus was a robust type; dated from 2 to 1.5 MYA.
• Australopithecus afarensis is based on many skeletal fragments (Lucy) dated at 3.18 MYA.

32.4  Evolution of Early Homo

1.     Fossils are assigned to the genus Homo based on the following traits:

a. brain size 600 cc or greater;
b. jaw and teeth are human-like; and
c.  tool use seems evident.

2.     Homo habilis and Homo rudolfensis

• The oldest fossils to be classified in the genus Homo aredated at around  2 mya.
• H. habilis and H. rudolfensis had a brain size as large as 8oo cc and smaller cheek teeth
• Cut marks on bones suggest the use of tools to prepare meat and possible scavenging.
• Tools associated with these two speciesinclude flakes used to scrape away hide or remove meat; they were likely omnivores.
• The skulls indicate that this hominid may have had speech
• Culture is dependent on the ability to speak and transmit knowledge; it is thought that the advantages of a culture to these hominids may have hastened the extinction of the austropithecines.

3.     Homo ergaster and Homo erectus

• Eugene DuBois, a Dutch anatomist, unearthed the first H. erectus bones in Java in 1891.
• Fossils found in Africa, Asia, and Europe date between 1.9 and 0.3 million years ago.
• The African and Asian types may be different species.
• H. erectus had a brain capacity of 1000 cc, was taller than H. habilis, and had a striding gait.
• H. erectus fossils found in Java and the Republic of Georgia at 1.9 MYA and 1.6 MYA indicates an early migration from Africa, followed by H. erectus evolving in Asia and spreading to other areas.
• These are the first hominids to use fire, fashion more advanced tools, to be systematic game hunters, and possibly to use home bases.
• Fossil remains of Homo floresiensis were discovered in 2004 on the island of Flores in the South Pacific; it was the size of a three-year-old human being but with a braincase only one-third the size.
• Researchers believe this species evolved from normal sized H. erectus, but underwent “island dwarfing.” 

32.5  Evolution of Later Homo

Two contradicting hypotheses are suggested about the origin of modern humans

1. The multiregional continuity hypothesis proposes that modern humans originated separately in Asia, Europe, and Africa.

a. If valid, then a distinctive continuity in anatomy and genetic variation is expected in each location.
b. Evolution of modern humans would be essentially similar in several different places.

2. The out-of-Africa hypothesis states that modern humans originated only in Africa and after migrating into Europe and Asia, they replaced the archaic Homo species found there; current evidence leans toward this hypothesis.

a. All extant humans are descended from a few individuals from about 100,000 years ago.
b. Mitochondrial DNA analyses indicate a close genetic relationship among all Europeans; although the first analysis was flawed, the data tend to support the out-of-Africa hypothesis.

A.    Neandertals

1. Neandertals were named for Neander Valley in Germany where skeletons were dated as early as 200,000 years ago.
2. Neandertals are classified as Homo neandertalensis.
3. Classic Neandertal anatomy includes massive brow ridges; a nose, jaws, and teeth that protruded forward; a low sloping forehead; a lower jaw sloping back without a chin; a longer pubic bone; a slightly larger brain than that of modern humans; shorter and thicker limb bones; and heavier muscles in the shoulder and neck.
4. It is speculated that a larger brain than that of modern humans was required to control the extra musculature.
5. The sturdy build of Neandertals was likely an adaptation to cold climate; they lived in Eurasia during the last Ice Age.

6.     The Neandertals give evidence of being culturally advanced.

a. Most lived in caves, but those who lived in the open may have built houses.
b. They manufactured a variety of stone tools, including spear points, scrapers, and knives.
c.  They used and could control fire, which probably helped in cooking frozen meat and in keeping warm.
d.  They buried their dead with flowers and tools and may have had a religion.

B.    Cro-Magnons

1.  Cro-Magnons are the olderst fossils to be designated H. sapiens; they werefound in Eurasia 100,000 years ago.
2.  Cro-Magnons are named for a fossil location in France and had a thoroughly modern appearance.
3.  They had advanced stone tools and may have been the first to throw spears.
4.  Cro-Magnons hunted cooperatively, and perhaps were the first to have had a language.
5.  They may have been responsible for the extinction of large mammals during the late Pleistocene.
6.  Cro-Magnon culture included figurines carved out of bone and antler, and cave paintings.

C.    Human Variation

• Some human variation evolved as adaptation to local environmental conditions: darker skin to protect from UV light, lighter skin for vitamin D production, etc.
• A bulkier body also benefits in colder regions while hot climates favor a slight build and longer limbs.
• Hair texture, eyelid fold, and other traits are not explained as adaptations.
• Variation among modern populations is considerably less than among archaic human populations of 250,000 years ago.
• Comparative studies of mDNA indicate that human populations had a common ancestor no more than a million years ago.
• The great majority of genetic variation, about 85%, occurs within ethnic groups, not among them.
• Genetic Evidence for a Common Ancestry
• The multiregional hypothesis suggests that different human populations came into existence as long as a million years ago, giving time for ethnic differences to accumulate despite gene flow.
• The out-of-Africa hypothesis suggests that modern humans have a relatively recent common ancestor who evolved in Africa then spread to other regions.
• Studies with mitochondrial DNA show that differences among human populations are consistent with their having a common ancestor no more than a million years ago..