Evolution of The Horse - Research Paper Example

Running Head: EVOLUTION OF THE HORSE The modern horse- Equus caballus is one of 9 surviving species of the genus Equus. Its surviving relatives include species of Zebra and Wild Asses. Equus caballus is believed to have descended from Hyracotherium, a small dog-sized genus that existed 55 million years ago (mya). Fossil and DNA records show that the phylogeny of the genus Equus has not been a simple linear descent of ever increasing size and horse like characterizations. The evolution of Equus caballus has rather, been a story of microevolution, with fluctuating sizes, varying characteristics, species interbreeding and migrations across the old and new worlds (Hunt, 1995). Phylogeny of the Horse [Equus caballus] Hyracotherium, the earliest relative of the modern horse lived 55mya, during the Lower Eocene epoch. Around 150 known fossil species descended from the Hyracotherium, most of which became extinct and resulted in an evolutionary dead-end. This paper shall focus upon major genera of the supposed line of ancestral descendance from Hyracotherium to the modern horse. No references are made to tangential genera. 1. Palaeothere Group – Foliage and fruit browsing perissodactyls Hyracotherium: The genus Hyracotherium lived during the lower to middle Eocene. It was a small dog-sized mammal with four toes on its forefeet and three on the rear. It had low-crowned teeth with horse-like ridged molars. The face was short with centred, forward facing eye sockets and a short distance between the front and cheek teeth [diastema]. Its physical characteristics suggest that it inhabited forested areas and browsed upon foliage (Garner, 2005 ; Tufts University, 1998). Orohippus: Appears during the middle Eocene, circa 2mya after Hyracotherium and existed from 52-45mya. It is believed that Orohippus evolved from a species of Hyracotherium but different species from the two genera co-existed during this epoch. Orohippus characteristics were much the same as Hyracotherium bar the vestiges of the 1st and 2nd toes, which vanished (Tufts University, 1998 ; Hunt, 1995). Epihippus: Appears 47mya in the middle Eocene. Epihippus characteristics were alike the previous genera bar a continuing evolution of the teeth. The rear premolars had evolved to become like molars, suggesting that the Epihippus was eating more hard foliage (Hunt, 1995). Mesohippus: Mesohippus was a sheep-sized descendant of the Orohippus that became extinct sometime in the middle Oligocene and lived circa 37-32mya. It too was a browsing animal with low-crowned teeth, but had evolved to have only three toes on its forefeet. Its cranial capacity had also noticeably increased (Hunt, 1995). Miohippus: This species existed from 32-35mya and developed a variable extra crest on its upper cheek teeth that became characteristic of its descendants. Miohippus speciated from Mesohippus and the two species co-existed for 4 million years. Over twelve known distinct genera descended from the Oligocene Miohippus that (Tufts University, 1998 ; Hunt, 1995). Parahippus: The Parahippus existed from 24-17mya. The genus is thought to be the evolutionary link between Palaeothere and Hipparion groups. The genus still possessed three toes but the side toes became smaller. The heads were long or “horse faced” with eye sockets placed further to the sides of the skull than previous genera (Tufts University, 1998). 2. Hipparion Group – Grazing perissodactyls, Equidae family Merychippus: The Merychippus genus was present from 17-11mya during the Miocene. It is in the Merychippus that characterizations of the modern horse began to appear. Firstly they had high-crowned teeth, coated in cement and with a greater complexity in the chewing surface than their predecessors; suggesting that they had become grazing animals. Secondly, the central toe appears to have born most of the weight, and appears to have had a strong elastic ligament passing behind it (Garner, 2005). The face had become elongated and the legs far longer than earlier genera (Tufts University, 1998). Pliohippus: The genus Pliohippus was present from 12-6mya during the late Miocene to Pliocene epochs. Pliohippus possessed high-crowned teeth designed for grazing and in some species, the outer toes evolved to become vestigial. The genus evolved from Merychippus but species from the two genera co-existed (Tufts University, 1998). Pliohippus was previously thought to be a direct ancestor of the Equus lineage but subsequent research has shown that significant Pliohippus characteristics of facial fossae and curved teeth are not present in Equus (Macfadden, 1984). Dinohippus: Present from 13-5mya during the late Miocene to Pliocene epochs the Dinohippus is believed to be the vestigial toed predecessor of Equus. The exact ancestor of the Dinohippus is as yet unknown Equus: The genus Equus appeared 4mya during the Pleistocene epoch. Its characteristics were typically hose-like: rigid spine, fused leg bones, long legs, single toe with side ligaments, long face, flexible muzzle high-crowned and straight crested teeth with strong cement, and a slightly enlarged cranial capacity. Equus migrated throughout Asia, Europe and the Americas, with species evolving into the modern Zebra, Horse and Wild Ass. It later became extinct in the new world. Equus species retain the gene for producing side toes and “deformed” offspring have been known to be born (Hunt, 1995). Equus caballus: Glacial periods of the Pleistocene epoch resulted in mass extinctions of many species, including those of the Equus genus. During the last glacial period 10,000 years ago before present (BP) Equus caballus found refuge in an area the Iberian Peninsula. It is here that the modern horse, as we know it developed (Seco-Morais, Oom, Quesada & Matheson 2007). References Garner, P. (2005). Horse Evolution by Paul Garner BSc (Hons), FGS Retrieved from http://www.truthinscience.org.uk/site/content/view/55/65/ Hunt, K. (1995). Horse Evolution Retrieved from http://www.talkorigins.org/faqs/horses/ horse_evol.html MacFadden, B. J. (1984). Astrohippus and Dinohippus from the Yepomera local fauna (Hemphillian, Mexico) and implications for the phylogeny of one-toed horses. Journal of Vertebrate Paleontology. 4(2), 273-283. Seco-Morais, J., Oom, M.M., Quesada, F. and Matheson, C.D. (2007). Ancient Iberian horses: a method to recover DNA from archaeological samples buried under sub- optimal conditions for preservation [Abstract]. Journal of Archaeological Science, 34(10), 1713- 1719 Tufts University. (1998) Horse Evolution Over 55 Million Years. Retrieved from http://www.flmnh.ufl.edu/natsci/vertpaleo/fhc/firstCM.htm Abstract for submission as electronic copy: Journal: Journal of Archaeological Science, Volume 34, Issue 10, October 2007, Pages 1713-1719 Title: Ancient Iberian horses: a method to recover DNA from archaeological samples buried under sub-optimal conditions for preservation Authors: J. Seco-Morais, M.M. Oom, F. Quesada and C.D. Matheson URL: http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6WH8-4N1SPD5-1&_user=10&_coverDate=10%2F31%2F2007&_rdoc=1&_fmt=high&_orig=search&_origin=search&_sort=d&_docanchor=&view=c&_searchStrId=1562857547&_rerunOrigin=google&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=70da7430e4f90ca3f90d3b4e9366504e&searchtype=a Abstract Existing methods to extract, amplify, and sequence ancient DNA (aDNA) from horse bone and teeth were optimized to recover DNA from a depositional environment of highly permeable acidic soil. DNA was successfully retrieved using 0.10g of bone powder from horse (Equus sp.) remains dating to 25 K years utilizing the methods optimized for this archaeological material. The genetic analyses were performed in a facility that is dedicated to ancient DNA research (Paleo-DNA Laboratory, Thunder Bay, ON, Canada) and has not been previously used to analyse modern or ancient horse DNA. Research was replicated to obtain reliable sequencing results for six samples from the Iberian Peninsula that were consistent with published sequences of Equus caballus. The archaeological sequence data obtained support hypotheses that promote the significance that the Iberian Peninsula has had to the multi-focal centres of origin for horse domestication and distribution of modern horse breeds. The data presented may provide evidence of the existence of an Iberian refugium for Equus during the last glacial period, 10 K years BP. Further molecular data analyses will enhance the ideas presented by this data and our understanding of horse domestication and phylogeny. The optimization of molecular techniques to successfully obtain DNA using minimally destructive, cosmetically sensitive techniques from archaeological remains endeavours to foster further cooperation between museums and researchers. Keywords: Ancient DNA; Iberian Peninsula; mtDNA; Horse evolution; Horse domestication; Methodology Read More