Chapter 17: Human Evolution

17-1 The Study of Human Origins

17-2 Fossil Evidence of Hominid Evolution17-3 Hypotheses of Hominid Evolution

I. The Hominid Fossil Record (understanding HUMAN evolution)

• (1) Revealing our ANCESTRY through FOSSIL records, and

• (2) Investigating closest of our living kin (primates).

17-1 The Study of Human Origins

NOTE: Some human behavior AND morphology are SIMILAR to those of other PRIMATES, HOWEVER some remain UNIQUELY human.

(1) Paleoanthropologists (a.ka., biological anthropologists)• Uncover FOSSIL EVIDENCE (including TOOLS, bones of PREY, and PLANT remains) used to support LIFESTYLES of human ancestors.

(2) Hominids (hominid evolution)• Modern humans AND our most recent ancestors;

NOTE: FOSSIL EVIDENCE largely pertains to skulls, teeth, bone fragments, AND also tools (in the more recently evolved hominids).

(3) Pelvis (anatomical EVIDENCE for hominid evolution)• Bones MAKING up the “HIPS” are evidence in SHOWING whether or not an organism can walk UPRIGHT (bipedalism).

II. Primate Characteristics (Order Primata, Class Mammalia)• 2 groups are RECOGNIZED…

(1) Anthropoid primates

(2) Prosimian primates

• NOTE: ALL primates evolved GRASPING hands, acute COLOR vision, FRONT-facing eyes (depth perception) & proportionately LARGE brains.

(1) Anthropoid Primates (majority are GROUND species)• MORE-RECENTLY evolved e.g., marmosets, gibbons, monkeys, apes, and humans.

(2) Prosimian Primates (majority of species are ARBOREAL)

• MORE-PRIMITIVE and ANCESTRAL primates e.g., lemurs, lorises, and tarsiers.

(3) Prehensile (adaptation of primate DEXTERITY)• Movable fingers and toes (GRASPING using hands and feet) ancestral CLAWS have become MODIFIED as flattened NAILS.

(A) Characteristics of Anthropoids (rotating shoulder and elbow joints) • Better DEXTERITY, LARGER and more COMPLEX brain (than prosimians), IMPROVED color vision (evolved with being DIURNAL).

(1) Opposable Thumb (human evolution LOST opposable big toe) • 2-jointed thumb OPPOSITE to other fingers—allows for a NEW arrangement of fingers better for PRECISION.

(2) Dental Formula (humans and anthropoids SHARE this arrangement)• Includes 2 pairs of INCISORS, 1 pair of CANINES, 2 pairs of PRE-MOLARS, and 3 pairs of MOLARS.

(3) Great Apes (Modern Apes; Chimpanzee and Human = ~98% DNA)• Gibbons, orangutans, gorillas, and chimpanzees LARGER cranial capacity MORE complex structure (similar to HUMAN evolution).

NOTE: As primates evolved FROM prosimian TO anthropoid, brain SIZE increased, allowing for complex behaviors and BETTER adaptability)

(B) Characteristics of Humans (Morphological distinctions)

• We are distinguished FROM the great apes in SEVERAL ways, including…(1) Human jaw is ROUNDER rather than NARROW U-shaped like great apes.(2) Human pelvis is CUP-shaped rather than the FLATTER ape pelvis.(3) Human spine is S-SHAPED rather than the SINGLE curve of ape spine.(4) Human foot has SHORT, ALIGNED toes rather than the longer, grasping toes (and opposable, big toe) of the ape.

NOTE:

Characteristics 2-4 work together to support FULL TIME BIPEDALISM in humans; (ONLY MAMMALS CAPABLE OF BIPEDALISM).

(1) When analyzing fossils, scientists examine the foramen magnum, the opening for the spinal cord in the base of the skull. What does the location of the foramen magnum on a skull tell about the posture of an animal?

Critical Thinking

(1) Bipedalism (defines the Hominid lineage, from QUADRUPEDAL)• An adaptation to GRASSLANDS ability to walk primarily on TWO legs, a UNIQUELY human trait.

NOTE: An ENLARGEMENT of human BRAIN has been shown to hold extensive areas devoted to production and understanding of SPEECH—important for cultural and intellectual development.

I. The First Hominids (evidence of bipedalism separates PRE-hominids)• Hominid gene pool has EVOLVED towards a LARGER BRAIN and BIPEDALISM (e.g., improved vision, freed-up forelimbs).

17-2 Fossil Evidence of Hominid Evolution

NOTE: Fossils of hominids, UNLIKE those of apes (PRE-hominids) and their ancestors, show ADAPTATIONS for UPRIGHT walking.

(1) Quadrupedal (advantages VS. disadvantages)• Modern NON-human anthropoids walk on FOUR limbs (anatomy varied).NOTE: Recent fossil evidence (on A. afarensis) has SUPPORTED a belief that UPRIGHT walking evolved BEFORE any other adaptation (i.e., did NOT require a LARGE brain) making hominids UNIQUE among anthropoids.

II. Australopithecus (a hominid GENUS)

• 1974: 3 m.y.o. PARTIAL skeleton of a PRIMATE found in the Afar Valley of eastern Africa by Donald Johanson.

NOTE: Fossil named Australopithecus afarensis (A. afarensis)—southern APE of the Afar Valley. Fossil was nicknamed Lucy, (estimated to be 3.0-3.9 m.y.o.)

(1) Lucy (hominid fossil ~ 3.2 m.y.a., ~ 475 cc skull )• Smaller CRANIAL CAPACITY (than humans) ALSO with SHORTER stature than humans (~ Lucy was the HEIGHT of a chimpanzee).

(A) Other Australopithecines (different hominids COEXISTED (same TIME)

(DESCENDANT) A. africanus (2.3-3.0 m.y.a.)—A possible DESCENDANT of Lucy, taller/heavier with a slightly LARGER cranial capacity (~430-550 cc).(DESCENDANT) A. robustus and A. boisei (1.0-2.6 m.y.a.)—More RECENT descendant (BUT non-human) HEAVY and LARGE SKULLS and larger TEETH (~450-600 cc).

*(ANCESTOR) A. anamensis (~3.5 m.y.a. OLDER than Lucy?)—A possible ancestor of Lucy, found in 1995; presence of a SHINBONE may indicate *BIPEDALISM*.

(B) An Older Hominid? (1995 uncovered an OLDER hominid)

• A NEW GENUS that predates Australopithecus by 200,000 years—

Ardipithecus ramidus, a *BIPEDAL* and the EARLIEST FOUND hominid.• NOTE: Human evolutionists do NOT believe human evolution occurred in a SINGLE, uninterrupted parade of increasingly humanlike forms. • Rather evidence suggests SEVERAL hominid forms arose AND some died out, leaving NO descendants. Thus the HUMAN EVOLUTIONARY TREE contains many branches (like a BUSH).

I. Early Members of Homo, The Human Genus (20th century fossils)

• HUMAN-LIKE morphology resulted in NEW human genus, Homo (1) LARGER Cranial Capacity (700 CC), (2) TOOL Use.

17-3 Hypotheses of Hominid Evolution

NOTE: As RESEARCH continues, we are observing DEAD-END branches of human evolution AND evidence that DIFFERENT hominids COEXISTED.

(A) Homo habilis (“handy human,” tool MARKS on animal bones)• 1.6-2.5 m.y.a.: 1st to make tools and weapons (also have larger SKULL)

(B) Homo erectus (“upright human” 1.8 m.y.a. to 50,000 years ago)• Believed to be 1st hominid OUT of Africa fossils of “Java Man” found on Island of Java (Pacific) (also Europe, Asia).

NOTE: Compared to MODERN humans, H. erectus had a thick skull, large brow ridges, a low forehead, and large protruding teeth. To survive in COLDER climates of Europe and Asia, H. erectus may have used FIRES in caves for warmth and cooking.

II. Homo sapiens (include TWO lineages Neanderthal and Cro-Magnon)• Descended FROM H. erectus ~ 800,000 years ago.

(Existed until as recently as 50,000 to 35,000 years ago).

NOTE: When H. sapiens AROSE, it is possible they did NOT replace H. erectus RIGHT AWAY. Findings (fossils) show coexistence of BOTH SPECIES may for occurred for ~ 700,000 years.

(A) Neanderthals (LARGEST C.C., evidence of STONE TOOLS for skins)• EARLIEST group of H. sapiens from around 230,000-30,000 y.a. and were found in parts of Europe and Asia.

NOTE: Heavy bones, THICK brow ridges, protruding teeth, AND a very LARGE cranial capacity (larger than modern humans?!)—Are NOT believed to be ancestral to modern humans and DISAPPEARED (LEFT NO DESCENDANTS) at the same time that MODERN H. sapiens evolved in LARGE numbers. (possible tribal warfares OR disease?)

(2) According to existing fossil evidence, Neanderthals died out about 30,000 years ago. Some anthropologists hypothesize that the Neanderthals were killed off by Cro-Magnons. Others hypothesize what the two groups interbred. What evidence would you look for to evaluate these two hypotheses?

Critical Thinking

(B) Homo sapiens (MODERN BETTER-developed tools/weapons)• 1st fossils to bear RESEMBLANCE to modern humans found in southwestern FRENCH CAVES ~ 35,000 years old. (Cro-Magnons)

NOTE: Cro-Magnons are distinguished from Neanderthals by their HIGHER forehead and RECESSED brow ridge and teeth; also stood TALLER at about 6 feet tall (ADVANCED use of TOOLS).

• OLDEST modern H. sapiens fossils are about 100,000 years old and were found in Africa—BUT as Neanderthals declined, modern H. sapiens became MORE advanced in hunting, habitat-building, and survival.

(3) How might the acquisition of language account for the very fast cultural and intellectual development that has occurred in the evolution of humans?

Critical Thinking

III. Spread of Modern Humans (TWO theories)• ALL modern humans belong to a SINGLE species, Homo sapien sapien; So HOW did all of our genetic VARIATION arise OVER TIME?

(4) From H. habilis to modern H. sapiens, there has been a trend for brain size to increase. Is it inevitable that any future descendant species of H. sapiens will have larger brains than we have? Explain your answer in terms of some form of natural selection, such as directional selection.

Critical Thinking

(A) Multiregional Evolution (LESS widely accepted)

• Gene FLOW existed AMONG different populations during TRANSITION from H. erectus H. sapiens.

(1) Multiregional Hypothesis of Human Evolution [1 MILLION years AGO]• PARALLEL populations of H. sapiens evolved SIMULTANEOUSLY from DIFFERENT H. erectus populations LOCATED around the WORLD.

(A) Out of Africa (MORE widely accepted, mitochondrial DNA)

• MODERN H. sapiens originated about [100,000 to 200,000] years ago and then, LIKE H. erectus before them, LEFT Africa to colonize the WORLD.(1) Recent African-Origin Hypothesis (supported by mitochondrial DNA)• MODERN H. sapiens evolved IN Africa and SPREAD throughout the world, REPLACING populations of H. erectus and EARLY H. sapiens.

(5) Compared with humans, apes do not have a well-developed voice box or well-developed facial muscles. How might the ape’s anatomy affect its ability to use spoken language?

Critical Thinking

Extra Slides AND Answers for Critical Thinking Questions(1) In social groups with language, information can be passed on and

accumulated, which significantly speeds up cultural and intellectual development.

(2) The location of the foramen magnum indicates the angle at which the spinal cord exits the skull, a clear indicator of posture.

(3) Fossil evidence of Neanderthals who met a violent death in areas where Cro-Magnons later flourished would constitute evidence that the Neanderthals were killed off by Cro-Magnons. Fossil evidence of a body type representing a blend of Neanderthals and Cro-Magnon features would support the hypothesis that the two groups interbred.

(4) It is not inevitable that descendant species of H. sapiens will have larger brains that modern humans. Stabilizing selection tends to keep traits in an average range. The size of the human female pelvis must enlarge along with any enlargment of the head.

(5) The ape’s anatomy would restrict its ability to use spoken language because speed production requires a great degree of control of the voice box and facial muscles.

Revisiting Evolution

• Evolutionary theory holds that the organisms alive on Earth today, including humans, evolved from shared ancestors.

Assessing Prior Knowledge

• What theories are there about how new species form? And what are the criteria for becoming a new species?

• During their evolution, populations acquired characteristics that distinguished them from their ancestors and from other populations alive today.

• Distinguish between “species” and “race,” with respect to human populations.