Scientists discovered fossil evidence of mesohippus, an ancestor of the modern horse. Mesohippus lived in North America about 40 million years ago. Mesohippus was about two feet tall, ran on hooves with three toes, and had large, grinding teeth. It is believed that these early horses lived in the thick forests.

What may have helped horses change over the last 40 million years?
A) an increase in their height and weight
B) the coming and going of several ice ages
C) a change in their food, from trees to grass in the Great Plains
D) being captured by Indians and used for work, pulling wagons and riding

cells use diffusion to get substances in and out of themselves. that diffusion will occur across the cell membrane or the outer covering of the cell. because we are talking about the surface of the cell, surface area and volume are  important factors here. as a cell increases in size, its surface area and volume increase as well.  we call that the surface area to volume ratio (sa: vol).  but the catch is that  volume increases at a greater rate than surface  area. in other words, as a cell gets bigger, the sa: vol gets smaller. so a really small cell like bacteria is mostly cell membrane (surface) with very little contents (volume). a really big cell like an egg, is mostly insides with a super thin membrane. so what ends up happening is that the cell cannot efficiently diffuse all of the stuff it needs in/out to survive. therefore cell size is limited to about 100mm. so how do we make organisms bigger than 100mm? cells divide! by dividing and becoming multicellular, organisms are able to grow!   check out the diagrams i attached to see the effect of sa: vol. i hope this ! good luck!