Primates

Primates (from the Latin primas, "of the first rank," coined by Carl Linnaeus in 1758) is the order of placental mammals (Eutheria, Mammalia) that comprises all lemurs, lorises, galagos, tarsiers, New World monkeys, Old World monkeys, and apes (including humans, Homo sapiens). Encompassing approximately 520 described extant species across 16 families—ranging in body mass from Madame Berthe's mouse lemur (Microcebus berthae, 30 grams) of Madagascar to the eastern gorilla (Gorilla beringei, up to 200+ kg) of Central Africa—primates belong to the superorder Euarchontoglires, sitting within the clade Euarchonta as sister to Dermoptera (Southeast Asian colugos or "flying lemurs," the closest living non-primate relatives of primates) and Scandentia (treeshrews).

Diverging near the Cretaceous–Paleogene (K–Pg) boundary (~68–65 million years ago) alongside Paleocene stem-relatives (Plesiadapiformes† such as Purgatorius and Carpolestes), crown-group primates (Euprimates, represented in the earliest Eocene ~56–55 Ma by Teilhardina, Notharctus, and Archicebus achilles) evolved in tropical angiosperm forest canopies via fine-branch visual predation on insects and co-evolutionary dispersal of fleshy fruits and nectar (Matt Cartmill, 1974; Robert W. Sussman, 1991).

1. Prehensile Pentadactyl Hands and Feet with Opposable Hallux/Pollex and Flat Nails: Unlike cursorial mammals that reduced their digits and encased their distal phalanges in laterally compressed claws or hooves, primates retain the ancestral pentadactyl (five-digit) limb layout modified for arboreal grasping:
   • A divergent, pseudo-opposable or truly saddle-joint-opposable big toe (hallux) and thumb (pollex) (with the hallux secondarily realigned inline for bipedal striding only in hominins);
   • Replacement of curved keratinous claws by flat dermal nails (ungulae) on at least the hallux and pollex, backed by wide, sensitive apical tactile pads etched with friction-enhancing epidermal ridges (dermatoglyphics / fingerprints) and dense Meissner's corpuscles.
2. Orbital Convergence, Postorbital Bar/Closure, and Stereoscopic Color Vision: The left and right bony eye sockets (orbits) rotated forward toward the midline of the face (orbital convergence), creating a wide zone of binocular overlap that enables stereopsis (3D depth perception) essential for judging branch distances during leaping and reaching. To isolate the eye from mechanical deformation when chewing muscles (temporalis and masseter) contract in the adjacent temporal fossa, all primates possess a complete bony postorbital bar (in Strepsirrhini) or a fully walled postorbital septum/closure (in Haplorhini: tarsiers and anthropoids). Furthermore, whereas almost all non-primate placental mammals are red–green colorblind dichromats (retaining only the short-wavelength S-opsin on an autosome and a single medium/long-wavelength M/L-opsin on the X chromosome after the nocturnal Mesozoic bottleneck), catarrhine primates (Old World monkeys and apes) duplicated the X-linked opsin gene (~35–40 Ma) into separate green (M, ~530 nm) and red (L, ~560 nm) opsins, restoring full routine trichromatic color vision to detect ripe orange/red fruits and young reddish flush leaves against green foliage (Nathans et al., 1986; Dominy & Lucas, 2001)!
3. Petrosal Auditory Bulla and Encephalization: In all crown primates, the ventral floor of the bony middle-ear chamber (auditory bulla) is ossified exclusively from the petrosal bone. Simultaneously, primates exhibit the highest relative encephalization quotients (EQ) among terrestrial mammals, driven by dramatic expansion of the visual cortex (V1–V5) and granular prefrontal cortex, accompanied by a "slow" K-selected life history: long gestation periods, single births (twins in callitrichid marmosets/tamarins), prolonged juvenile brain myelination and social learning, and long adult lifespans.
4. Primate-Specific Alu Retrotransposon Elements: At the genomic level, every living primate (and no non-primate mammal!) shares a massive lineage-specific radiation of ~300-base-pair Short Interspersed Nuclear Elements (SINEs) derived from the 7SL RNA gene of the signal recognition particle—the Alu retrotransposon family (~1.1 million copies comprising ~11% of the entire human genome!), making Alu insertion polymorphisms a homoplasy-free cladistic marker across all primate nodes (Batzer & Deininger, 2002).

Modern phylogenomics replaces the paraphyletic historical grade "Prosimii" (which lumped tarsiers with lemurs) with two strictly monophyletic suborders based on nasal anatomy, placenta structure, and vitamin C biochemistry:

Why do Old World monkeys and New World monkeys have tails while all living apes and humans (Hominoidea) are completely tailless (having fused 3–5 vestigial caudal vertebrae into the internal coccyx)? In 2024, Bo Xia, Jef D. Boeke, Itai Yanai, and colleagues solved this 25-million-year-old evolutionary puzzle:

• Comparing the genomes of 6 hominoid species against 15 tailed monkey species across 140 tail-development genes, they discovered a hominoid-specific ~300-bp AluY retrotransposon insertion inside intron 6 of the TBXT (Brachyury) gene—the master transcription factor that patterns the embryonic chordamesoderm, notochord, and posterior tailbud.
• In ape and human embryos, this Newly inserted AluY element sits in inverted orientation directly across from an older ancestral intronic AluSx1 element. During pre-mRNA transcription, the two inverted Alu elements base-pair into a stem-loop hairpin, causing the spliceosome to skip exon 6 and produce an alternatively spliced TBXT^Δexon6 isoform—and when engineered into mice via CRISPR, co-expression of full-length Tbxt and Tbxt^Δexon6 produced completely tailless mice (along with a ~5% incidence of neural tube closure defects resembling human spina bifida)!

Why can lemurs synthesize their own Vitamin C while tarsiers, monkeys, apes, and humans get scurvy without dietary fruit?

Almost all vertebrates (including dogs, cats, mice, and strepsirrhine lemurs and lorises) synthesize their own L-ascorbic acid (Vitamin C) from glucose in the liver or kidney via the terminal enzyme L-gulonolactone oxidase (GULO). Around 60–65 million years ago, in the common ancestor of Haplorhini (tarsiers + monkeys + apes + humans), the GULO gene suffered multiple frame-shifting deletions and became a non-functional pseudogene (GULOP)—Neutralized because early frugivorous and insectivorous haplorhines consumed so much Vitamin C in their daily tropical fruit and leaf diet that losing endogenous synthesis carried zero fitness penalty, locking all descendants into an obligate dietary requirement for Vitamin C.

How did New World monkeys (Platyrrhini) reach South America when it was an isolated island continent?

During the Eocene Epoch (~40–34 million years ago), South America was completely isolated from North America and Africa by oceanic barriers. However, Paleogene fossils from Santa Rosa, Peru (Perupithecus, Bond et al., 2015) and Ucayali (Ucayalipithecus, Seiffert et al., 2020) show dental morphologies virtually identical to Eocene North African anthropoids (Proteopithecidae and Parapithecidae from the Fayum Depression of Egypt). Because the Eocene South Atlantic was narrower (~1,400–1,800 km across between West Africa and Brazil), paleocurrents and trade winds flowed rapidly from east to west, and sea levels dropped during the Eocene–Oligocene transition, ancestral monkeys rafted across the Atlantic on floating islands of storm-uprooted mangrove and riverine trees—accompanied in parallel by the ancestor of South American caviomorph rodents (guinea pigs, capybaras, chinchillas)!

1. Nathans, J., Thomas, D., & Hogness, D. S. (1986). Molecular genetics of human color vision: The genes encoding blue, green, and red pigments. Science, 232(4747), 193–202. doi:10.1126/science.2937147
2. Batzer, M. A., & Deininger, P. L. (2002). Alu repeats and human genomic diversity. Nature Reviews Genetics, 3(5), 370–379. doi:10.1038/nrg798
3. Perelman, P., Johnson, W. E., Roos, C., Seuánez, H. N., Horvath, J. E., Moreira, M. A. M., ... & Pecon-Slattery, J. (2011). A molecular phylogeny of living primates. PLOS Genetics, 7(3), e1001342. doi:10.1371/journal.pgen.1001342
4. Bond, M., Tejedor, M. F., Campbell, K. E., Jr., Chornogubsky, L., Novo, N., & Goin, F. (2015). Eocene primates of South America and the African origins of New World monkeys. Nature, 520(7548), 538–541. doi:10.1038/nature14120
5. Kuderna, L. F. K., Gao, H., Janiak, M. C., Kuhlwilm, M., Orkin, J. D., Bataillon, T., ... & Marques-Bonet, T. (2023). A global catalog of whole-genome diversity from 233 primate species. Science, 380(6648), 906–913. doi:10.1126/science.abn7829
6. Xia, B., Zhang, W., Zhao, G., Zhang, X., Bai, J., Brosh, R., ... & Yanai, I. (2024). On the genetic basis of tail-loss evolution in humans and apes. Nature, 626(8001), 1042–1048. doi:10.1038/s41586-024-07095-8