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Humans, living apes and monkeys are grouped together as anthropoid primates. The anthropoids share a common ancestor that lived sometime more than 55 million years ago. Paleontologists have found fossils of ancient anthropoids that lived very near that ancestor, and also ancient anthropoids that were related to tarsiers, the next closest branch of the primate phylogeny.
Among the anthropoids, Old World monkeys (cercopithecoids) and hominoids are closer relatives, and New World monkeys (ceboids) are more distantly related. In other words, cercopithecoids and hominoids form a group that descends from a common ancestor within the anthropoids. We call this group the catarrhine primates.
In this inquiry-based laboratory project, you will use evidence from two recent primates to infer the probable anatomy of their common ancestor. Together with your laboratory group, you will be assigned two recent primates. Every group will receive a different pair of primates. Some of these will be New World monkeys, some will be Old World monkeys, and some will be hominoids – that is, apes or humans. The pairs of primates will have one thing in common: their true common ancestors were ancient anthropoid primates.
How can scientists reconstruct the anatomy of a common ancestor, if they haven’t necessarily discovered fossils of that species?
Every inference about an ancestor is a hypothesis. If we see that two descendants of the ancestor are similar in their anatomy, we can begin with the hypothesis that the ancestor was also the same as those two descendants. For example, humans and squirrel monkeys (a New World primate) both have fingernails instead of claws. We can hypothesize that our common ancestor also had fingernails.
We test this hypothesis in several ways. We can look at other descendants of the same ancestor. Our common ancestor with squirrel monkeys was the same species as the common ancestor of gorillas and squirrel monkeys, and the same as the common ancestor of baboons (an Old World monkey) and howler monkeys (a New World monkey). Not only humans and squirrel monkeys but also gorillas, chimpanzees, baboons, macaques, guenons, langurs, howler monkeys, spider monkeys, and all other monkeys and apes have fingernails. It seems very unlikely that all these species would have evolved fingernails by coincidence or in parallel with each other. The hypothesis that they inherited fingernails from their common ancestor seems very well supported by this evidence.
We can also look at species that are more distantly related. Such species give an outgroup for our phylogenetic comparisons. Lemurs, tarsiers, and lorises all have fingernails also – although lemurs and lorises have instead a single claw, called a grooming claw on one finger. It appears from the evidence that not only the common ancestor of all anthropoids, but also the common ancestor of
Looking at an outgroup is especially important in cases where two living descendants of the same ancestral species are different from each other. Macaques have a tail. Humans don’t. Did our common ancestor – the ancestor of the catarrhines – have a tail or not? Looking at just catarrhine species doesn’t help us determine whether human ancestors lost a tail or instead macaque ancestors gained one. All living apes lack tails, all living Old World monkeys at least have some tail. But a look at an outgroup helps enormously. New World monkeys have tails, as do lemurs and tarsiers. These outgroups suggest that the ancestors of catarrhines had a tail and that the ancestors of the apes lost their tails.
For this inquiry, you will consider two major areas of anatomy. In week 2 of the laboratory, you will examine teeth. The number and anatomy of teeth vary among anthropoids, and you will develop and test a hypothesis about the number and anatomy of teeth in the common ancestor of your two primates.
In week 3 of the laboratory, you will examine body plan, including the number and types of vertebrae and the anatomy of the forelimb. You will develop and test a hypothesis about the body plan in the common ancestor of your two primates.
You will develop these hypotheses and work as part of a laboratory group. In week 4 of the lab, you will present your findings. One of the best aspects of this inquiry is that different groups may arrive at different hypotheses, depending on which species of recent primates they have examined. As you come together with other groups to discuss your findings, be prepared to compare the evidence from different groups to see whether it confirms or rejects your hypotheses about the common ancestor.