Essay on Blood: Equidae and Teeth

Submitted By elliebrown
Words: 790
Pages: 4

v2A M1 Explain the role of genes and the environment in evolution

Using an example from fossil evidence explain how the animals genes and their environment and the theory of natural selection have caused the animals to evolve.

Fossilised remains of dead animals and plants can be found in sedimentary rocks dating back up to 600 million years. These fossils especially bones and teeth give information about the dominant groups of living organisms at different stages in the development of the earth. Fossils also show how the body structure of individuals within certain groups changed over time, and these changes can often be more confidently related to environmental influence to which these organisms became more efficiently adapted.

A prime example of this is shown in the development of the horse, and especially the bones of the foot. The earliest ancestors of modern horses dated about 60 million years ago. The Eohippus was only 0.4 metres tall. With a relatively short head and neck and a springy, arched back. It had 44 low-crowned teeth. Eohippus browsed on soft foliage and fruit, probably scampering between thickets in the mode of a modern muntjac. It had a small brain, and possessed especially small frontal lobes. Its limbs were decently long relative to its body, already showing the beginnings of adaptations for running. However, all of the major leg bones were unfused, leaving the legs flexible and rotatable. Its wrist and hock joints were low to the ground. The forelimbs had developed five toes, of which four were equipped with small proto-hooves; the large fifth "toe-thumb" was off the ground. The hind limbs had small hooves. For a span of about 20 million years, Eohippus thrived with few significant evolutionary changes. The most significant change was in the teeth, which began to adapt to its changing diet, as these early Equidae shifted from a mixed diet of fruits and foliage to one focused increasingly on browsing foods.

40 million years ago, the second development of the horses this species was called Mesohippuis, it intermediate between the eohippus-like horses of the Eocene, (which don't look much like our familiar "horse") and more "modern" horses. The Mesohippuis lived in North America which became drier, and the earliest grasses began to evolve. The forests were yielding to flatlands, home to grasses and bushes. In a few areas it was covered with sand. In response to the change of environment, they began developing tougher teeth and becoming slightly larger and leggier, allowing for faster running speeds in open areas. The mesohippuis walked on three toes on each of its front and hind feet (the first and fifth toes remained, but were small and not used in walking). The third toe was stronger than the outer ones, and more weighted.

30 million years ago there was another major development in the horse evolvement. The Merychippus, had wider molars than its predecessors, which are believed to have been used for crunching the hard grasses of the steppes. Merychippus had a long face. Its long legs allowed it to escape from predators and migrate long distances to feed, it also had , side toes equipped with small hooves, but they probably only