INTRODUCTION Exercise 13 Hominid fossils (10 pts) (adapted from Petersen and Rigby 1999, pp. 221 225) The first significant hominid fossils were found north of Düsseldorf, Germany, in the Neander Valley in 1856. From then until now additional finds have expanded the fossil collections of the Family Hominidae. These fossils are among the most valuable objects of antiquity in the possession of humankind. Unlike most other fossil types, however, there are so few good hominid fossils that each new discovery can have profound importance for reconstructing the course of human evolution. Examples of single finds that revolutionized evolutionary thinking include the discovery of the Tuang Baby in Botswana in 1924, Homo habilis in Tanzania in 1962, Lucy in Ethiopia in 1974, Australopithecus footprints in Tanzania in 1976, Turkana Boy in Kenya in 1984, and the Black Skull in Kenya in 1985. Each find represents either an extraordinary bit of good luck by the finder, or, more typically, the result of deliberate, expensive, and painstaking excavations whose purpose was to seek hominid fossils at known productive sites. The most recent example of a spectacular new find is the 2001 discovery of a hominid cranium and associated fragments in Chad by the French paleoanthropologist Michel Brunet and his team. These fossils are dated at 6 to 7 million years old the oldest hominid yet found. This find undoubtedly will cause a wholesale revision of evolutionary scenarios, as previously the oldest known hominid fossils were ~4.4 million years old. Although there is no disagreement among scientists over the fact of human evolution, there is uncertainty with respect to detailed evolutionary relationships among the various known taxa. Modern hominids did not evolve from modern pongids (gorillas, chimps, orangutans), because both groups are contemporaneous. Clearly, however, modern hominids and modern pongids must share a common ancestor from which they diverged to follow their respective, independent evolutionary paths. The precise time of divergence is unknown, but it almost certainly occurred late in the Miocene Epoch, possibly around 7 million years ago. 13 1
The single greatest obstacle to understanding evolutionary relationships between hominids and pongids is inadequacy of the fossil record. There are only a few hominoid (i.e., hominid plus pongid; Fig. 1) fossils from the critical latest Miocene interval, 8 million to 5.3 million years ago. Morover, there are very few pongid fossils of any age, let alone from the interval immediately postdating their divergence from hominids. Evolutionary relationships between pongids and hominids will remain equivocal until new discoveries produce material from beds bracketing the time of divergence. Figure 1 Classification of primates. Hominids and pongids are related through common ancestry, and therefore are grouped together within the superfamily Hominoidea. (from Stanley 1998) 13 2
Figure 2 contrasts the skulls and upper jaws of modern man and modern chimpanzee. Note differences with respect to the following six features: cranial capacity, brow ridge, jaw, diastema, palate and dentition. Obviously, the most recent common ancestor to humans and chimps did not possess anatomy identical to either modern human or modern chimps; rather, both humans and chimps have diverged from the common ancestral state. For purposes of this exercise, however, we will examine undated fossil hominid skulls and jaws and attempt to arrange them in proper stratigraphic order on the basis of their similarity to modern humans and chimps. The assumption here is that a chimp-like hominid fossil is probably older than a modernlooking one, because its chimp-like features would place it closer to the most recent common ancestor to both modern chimps and modern humans. Figure 2 Comparison of skull and jaw features between modern man and modern chimpanzee. (from Petersen and Rigby 1999) 13 3
Part 1. Examine the drawings of six fossil skulls in Figure 3, noting similarities and differences between each fossil skull and modern human and chimp skulls. The age of these fossil skulls is known, but not given here. Reconstruct what you think to be the most likely stratigraphic sequence of the fossil skulls on the basis of their various features. Label the individual skulls from 1 to 6, with 1 being most chimp-like and 6 being most like modern man. Figure 3 Selected fossil hominid skulls and their dentition (the jaw is to the left of its associated skull). (from Petersen and Rigby 1999) Part 2. You probably discovered that ordering these skulls from most chimp-like to most human-like is a highly subjective process that requires some arbitrary 13 4
decisions. The procedure of ordering makes up a subdiscipline within the science of taxonomy. An approach that eliminates some, but not all, of the taxonomic subjectivity involves quantifying the morphological characters on which the evolutionary order of appearance is inferred. For example, a numerical value can be assigned to each important feature of each skull in order to quantify the degree to which each skull resembles modern man or modern chimpanzee: Example Table 1 (skull of a modern chimp). 1 1 1 1 1 1 According to this kind of scoring, a modern human would be assigned 18 points (3 points for each of six features) and a modern chimp 6 points (1 point for each of six features). Evolutionary stages in human evolution could be inferred by this approach, known as numerical taxonomy. a. Complete a table like Example Table 1 for each of the six skulls and then determine the order of evolutionary appearance on the basis of resulting scores. Skull A 13 5
Skull B Skull C Skull D Skull E 13 6
Skull F b. Give the order from most chimp-like to most human-like: c. What is the effect of poor fossil preservation? In other words, if a fossil isn t sufficiently complete to allow judging its similarity to a chimp or human, how does this affect the numerical score? Part 3. The art of taxonomy is in determining which taxonomic features are more useful than others in evolutionary studies. For example, an experienced evolutionary biologist or paleontologist might have reason to believe that differences in cranial capacity are taxonomically twice as important as differences in dentition. a. Select one or two morphologic features that you feel are taxonomically important and repeat the scoring. This time, however, double the point values assigned to the important features while keeping all the other point values the same as before. 13 7
Skull A Skull B Skull C Skull D 13 8
Skull E Skull F b. Give the new order from most chimp-like to most human-like: c. Which feature or features did you weight? Did you arrive at the same result as in Part 2? If not, how are the new results different? 13 9