MIOCENE (23 to 5.3 MYBP)
***Apes are MORE common and diverse than monkeys during the Lower and Middle Miocene (reverse of situation today)
***Apes evolved in Africa, which was separated from Eurasia during the Lower Miocene
*Africa and Eurasia reconnect at 18 MYBP, allowing apes to radiate into southern Europe and Asia
*Lower and early Middle Miocene climate is much warmer and wetter than today, with widespread tropical and warm-temperate forests. This was ideal ape habitat.
-The earliest ape fossils (the “Dental Apes”) are differentiated from monkeys by:
having the ape dental pattern of 4 offset cusps on the maxillary (upper) molars and 5 cusps (Y-5 pattern) on the mandibular (lower) molars
*LOWER MIOCENE APES = DRYOPITHECINES, within this overall category the European examples are mostly genus Dryopithecus
*The African Dryopithecines are called “Proconsulids” and most, but not all, are in the genus Proconsul
-The genus Proconsul is the first fossil with predominately ape characteristics
-Most of these early apes walked like monkeys (arboreal quadrupeds) rather than brachiating like modern apes
*First brachiators (?) fossil Morotopithecus (Uganda, 20 MYBP) Scapula may show shoulder designed for brachiation
*Best Proconsulid site: Rusinga Island (Lake Victoria, Kenya)
*The best documented Miocene ape is Proconsul africanus (18 MYBP)
*1. Lightly built skull, lacking crests
*2. Cerebrum larger than monkeys
*3. Typical ape dental characteristics including 5 cusps on all three mandibular molars
*4. P. africanus is an arboreal quadruped, and NOT a knuckle-walker like modern apes. Like monkeys, its legs and arms are equal in length
*In other words, proconsul africanus has an ape-like skull on a more monkey-like body
***Following these Lower Miocene apes, there are 3 major groups of MIDDLE MIOCENE APES:
*1. Sivapithecines- Asian great apes: Ancestors of the orangutan
*2. Gigantopithecines- Extinct giant Asian ape, diet = bamboo
*3. Oreopithecines- A catch-all category of various species of African and European apes including some possible ancestors of the human line
HOW DO YOU TELL A HOMININ FROM A HOMINOID
***Sine the hominin line may have emerged in the Upper Miocene, and because the earliest members of the hominin line are likely to be so ape-like that they will be VERY difficult to identify, it is necessary to DEFINE THE EVOLUTIONARY TRENDS THAT DIFFERENTIATED HUMANS FROM APES in order to help us determine exactly which fossil is the earliest hominin
*In the 1950s, systematic comparisons of humans and African apes were done to identify differences in anatomy and behavior.
*Assumed that apes have undergone LESS CHANGE from the ancestral state in anatomy and adaptation
*SO, since apes are more “primitive,” these differences could be used to define “hominin evolutionary trends” that could be used to identify the earliest hominins.
THE HOMININ EVOLUTIONARY TRENDS ARE:
1. Upright posture and bipedal gait
-Contrasts with knuckle-walking posture of gorillas and chimps
2. Reduction in the size of the canine tooth and a proportional increase in the size of the premolars and molars
-Includes an overall reduction in size of all teeth relative to body size
-There is also a change in the shape of the dental arcade from the straight premolar and molar rows of apes, to a smooth parabolic curve in humans
3. Changes in the form of teeth
-Canine changes from a long, conical shape to a short incisor-like blade
-The premolars and molars develop shorter and rounder cusps to permit side to side chewing
-All teeth, particularly the premolars and molars develop THICKER ENAMEL
-All of these first three trends were linked to a terrestrial adaptation
4. Tremendous increase in relative brain size (i.e. “encephalization”)
-Chimp brain today averages c. 350 cc.
-Human brain today averages c. 1,400 cc.
-Most of this is in the cerebral neocortex, the area where higher mental functions occur
5. Infants born “early,” develop slowly, and have long juvenile period
-Great apes reach sexual maturity at c. age 4-5, humans at c. 12-14
***Originally, we thought these five trends developed SIMULTANEOUSLY and SLOWLY, and that they were all linked to a single process/scenario. This scenario is:
***Apes came out of the trees and evolved bipedalism and the other traits all the same time as part of a mixed diet of TOUGHER (drier) PLANT FOODS and MEAT obtained by deliberate hunting
*Brain size increase was assumed to have been caused by this more predatory and technological adaptation.
BUT… Well dated evidence today shows that this scenario is only partly correct.
*Upright posture may appear as early as 6+ MYBP
*Dental changes also start over 6 MYBP
*Significant changes in brain size start at c. 3 (?) MYBP
*Meat-eating and stone tool use begin before c. 2.6 MYBP, but are not common until AFTER 2 MYBP
***SO only the posture and dental characteristics can be useful in identifying the EARLIEST hominins… BUT this only works if we have CORRECTLY identified what the characteristics of the last common ancestor of humans and apes were.
ASSUMED that the last common ancestor was something like a chimpanzee in its anatomy and adaptation. That MAY BE TRUE, but recent discoveries suggest it may not be!
*Middle to Upper Miocene (most fossils from 12-8 MYBP)
*Includes Oreopithecus bamboli, an Italian species that was a TRUE BRACHIATOR, but its dentition does not link it to any modern ape species or to the human line.
*Kenyapithecus wicker: an Oreopithecine from Africa
-Discovered by Louis Leaky at Ft. Ternan, Kenya
-Date c. 10-12 MYBP
-Fragmentary dental elements from a single individual
-Teeth include a crowded molar row and slightly thicker dental enamel than apes, so it MAY show the beginning of the hominin evolutionary trends, but it is probably too early in time, and there is not enough of the fossil to be certain
-Understanding human evolution is complicated by the “UPPER MIOCENE FOSSIL GAP” a period from 10 TO 5.5 MYBP which is represented by only a few fossil bearing deposits in Africa
-Can “MOLECULAR CLOCKS” help?
*Humans and Pygmy Chimps share about 98.5% of their DNA
*Beginning in the 1960’s, molecular biologists (Sarich & Wilson invented the technique) argued that by measuring the genetic difference between two species, you could ESTIMATE WHEN those species last shared a common ancestor (a “DNA clock”)
*This technique ASSUMES A UNIFORM MUTATION RATE OF DNA
The molecular dates for key events in human evolution are:
*Monkey-Ape split at over 23 MYBP (agrees with fossil record)
*Asian ape-African ape split: 17 to 12 MYBP (apes expanded out of Africa at c. 18 MYBP, so this is about right)
*Gorilla-chimp/human split: 11 to 8 MYBP (No fossil record to check this, but probably OK)
*Human-Chimp split: 7.7 to 5.5 MYBP (Fossil record seems to support this because a definite hominin line dates to at least 5 MYBP)