The primate family tree, also known as a phylogenetic tree, is long and complicated. By observing certain characteristics which an organism has in relation to other organisms, researchers can begin to classify both extinct and extant species into groups (also referred as taxa). This practice of grouping organisms based on shared characteristics is known as taxonomy. Developed by Swedish botanist Carolus Linnaeus in his 1735 book Systema Naturae, Linnaeus used physical characteristics and genetics to identify and differentiate between organisms. Linnaeus created a hierarchy of taxonomic groups to distinguish the amount of similarity between each organism. This hierarchical order can be memorized using a mnemonic device such as “Dear King Phillip Come Over For Good Soup”- Domain, Kingdom, Phylum, Class, Order, Family, Genus, & Species. Linnaeus’s taxonomic system is still used to this day. Each and every organism that you see in the world unless it hasn’t been discovered yet is classified by each of these taxonomic groups.
So, if mom and dad aren’t chimpanzees and grandma isn’t a tarsier, where do we place our human selves on the primate family tree? Biological anthropologists classify primates through varying sets of taxonomic characteristics based on the hierarchical level from Linnaeus’s classification system: kingdom, phylum, class, order, family, genus, species. However, researchers have added more specific taxonomic groups since the establishment of Linnaeus’s taxonomic system. These tend to focus more on the genetic portion of classification, something Linnaeus didn’t really have access to at the time. These groups include semiorders, suborders, infraorders, superfamilies, subfamilies, tribes, subgenera, and subspecies. Both you and all other living and extinct primates appear in one of the many clusters of branches located under the class Mammalia on the phylogenetic tree.
Our branch starts at the level of order and expands out all the way to the smallest branches on the tree-subspecies (15&20). However, belonging to the primate order is quite exclusive. Any organism who wishes to join the order Primates must have the following traits.
- Petrosal bulla
- Intratympanic portions of the facial nerve and carotid circulation
- Convergent orbits (forward facing eyes)
- A postorbital bar
- Flattened nails
- The ethmoid bone is exposed in the orbit
- An opposable hallux (thumb & big toe)
- An anterior elongation of the calcaneus (heel bone)
- A well-developed groove for the tendon of the flexor digitorum fibularis on the plantar surface of the calcaneal sustentacular process.
With the exception of the petrosal bulla, these features are not found exclusively in primates, however, the combination of these features is (27). Once we’ve passed the primate order, we must then pass the Haplorrhini semiorder, and Anthropoidea suborder. Since Haplorrhines occupy a broader category on the phylogenetic tree (semiorder), the traits or synapomorphies used to classify them will be more numerous than those used to classify Anthropoids, which occupy a narrower category (suborder). Haplorrhini are often diagnosed by soft tissue, dental, and cranial synapomorphies, whereas Anthropoids are often diagnosed by cranial, dental and postcranial synapomorphies. The synapomorphies for Haplorrhines and Anthropoids are listed below.
- Soft tissue features include: loss of nasal rhinarium (wet nose), loss of tapetum lucidum, development of a retinal fovea and development of a hemochorial placenta
- Dental features include: orientation of cristid obliqua and depth of the hypoflexid of the first molar
- Cranial features include: loss of the ethmoid recess in nasal region and the development of a post-orbital plate
- Cranial features include: a fused frontal, fused mandibular symphysis, complete postorbital closure, the lacrimal bone located in the orbit
- Dental feature includes: the posterior-most premolar is always a semimolariform tooth with a differentiated trigonid and talonid
- Postcranial feature includes: Flattened nail always on the second pedal digit
Following the Anthropoidea suborder, we then pas the Catarrhini infraorder which includes all Old World Monkeys and Apes. Following Catarrhini, we finally reach the branch of the phylogenetic tree closest to our own Hominoidea superfamily (hominoids), which includes all apes: Humans, Gorillas, Chimpanzees, Bonobos, Orangutans and Gibbons (15&31). While Hominoids are less taxonomically diverse than Catarrhines, they do share a common dental formula of 18.104.22.168 and both possess a tubular ectotympanic bone. However, compared to Old World monkeys, hominoids show relatively primitive molar teeth, with rounded cusps. Additionally, hominoids are characterized by broad palates, broad nasal regions, large brains, and no tail. Although Hominoids include all apes, another further division is made to reach us. The Hominidae family only includes the great apes (Gorillas, Orangutan, Chimpanzees, & Bonobos) and humans, allowing researchers to further distinguish differences among primate groups. Passing the Hominidae family branch, we finally arrive at our branch, the Homo genus. While there is only one species currently in the Homo genus, H. sapiens (that’s us!), it also includes all human ancestors. So, while your ancestral family tree might seem long and distant, it’s, in fact, very compact when compared to your phylogenetic primate family tree!