Ask A Scientist
Where do animals come from?
Asked by: Emma Kupiec
School: Glenwood Elementary School
Teacher: Amanda Herzog
Hobbies/Interests: Going outside
Career Interest: Teacher
Answer from Stanley N. Salthe
Research area: Natural philosophy Interests/hobbies: ecology, evolutionary biology, semiotics, systems science and thermodynamics. Woodland gardening, nature walks, all of the arts Ph.D: Columbia University Family: wife Barbara, two children Becky and Eric
Animals, like all living things, evolved from another kind of living thing. So the question refers back to another: ‘where did living things come from’? Currently scientists are trying to discover how life might have evolved from non-life on the earth long ago. We know that the first living things were simple cells like bacteria. These lived in water, in colonies on the surface of rocks or in mud, using chemicals or sunlight for energy and absorbed minerals for food. These evolved into simple plant-like things that used sunlight to photosynthesize food. The difference between these and animals is that animals consume other living things. In order to do this, they have to move about from one place to another as they use up the food in one location.
The ability to move independently is a significant feature of animals. This movement requires muscles and a nervous system to allow them to taste water and locate food. The ability to move requires that there be a front end, and necessarily then, a back end too. At the front end food ingested and at the back end waste products are ejected. A few kinds of animals do not have a front and back, like jellyfish and hydras. They wait for food to come to them, and eject used up food through their mouth. Like other animals these do move about from place to place, slowly.
The earliest animals ate plantlike beings, then evolved to be able to eat their own dead as well, becoming carnivores. From here it was a short step to evolve into predators that went about hunting other animals. Generally today predators hunt plant-eating animals – herbivores -- but modern herbivores were the last kind of animal to evolve. And so the earliest predators ate other carnivores, probably at first only dead ones, so they were detrivores. It took many eons for animals to become plant eaters, because they need bacteria in their gut to digest the cellulose surrounding plant cells, and this arrangement required time. Most modern land plants had evolved their cellulose making to protect them from being eaten. So, in short, animals came from early plantlike things.