Where do we come from? By "we" I mean humans in the large spectrum of life. What makes each of us different from every other human, but more like every other human than like any other animal roaming the earth, and, at the same time, somehow connected to all forms of life?
There are different explanations of our origins. According to the Biblical account, in the beginning God created all land creatures more or less at once (except humans, who were said to be created separately and specially). A 2010 Gallup Poll indicates that a majority of people in the U.S. with more than a high school level of education accept a different view, one in which present day animals (including humans) are believed to have evolved gradually from ancestral animals. While some believe God guided this process, others don't, but in either case they accept that human origins can be traced to earlier life forms. What they accept, of course, is Darwin's principle of evolution by natural selection.
Prior to Darwin, it was common to classify animals in a linear sequence. Aristotle had proposed a scala natura, a rank order of animals in terms of perfection, by which he meant similarity to humans. He had humans at the top of the scale, and formless creatures like sponges way down below, and all other organisms that he knew about in between. Christian theologians later refined this notion, changing the meaning of perfection. In their Great Chain of Being, God was at the top, followed by angels, then humans, and all the other life forms in descending order. But according to Darwin, animals (including humans) cannot be arranged in a linear sequence. He proposed instead that animals are more like points along the branching limbs of a tree, what he called the tree of life (that's right, Terrance Malick didn't invent the phrase).
Modern genetics has greatly aided the process of animal classification. Biologists today accept a version of the tree metaphor (a version with fewer major branches) for organisms with eukaryotic cells, that is, all the creatures we call animals. If we trace human origins backwards towards the major branch and then the trunk, we get a clear picture of which animals are in our past and which are not.
First of all, we are vertebrates, animals with a vertebral column that surrounds the spinal cord, and a bony skull case that houses our cephalic ganglion, or brain (cephalic has to do with the head, and a ganglion is a collection of cells). All mammals, birds, reptiles, amphibians, and bony fish are vertebrates. Humans are mammals, specifically primates, and like all mammals our ancestors were a particular kind of reptile that is no longer with us. Their ancestors, in turn, were bony fish. So where did the original bony fish, the first vertebrates, come from? The simple answer is, from invertebrates. But there's more.
Animals can be classified on an embryological principle, that is, a principle about the course of individual development. In some animals the mouth develops before the anus, and in others the anus develops first. The mouth-first organisms include numerous invertebrates, such as insects, slugs, snails, and flatworms. Vertebrates, including humans, are of the anus-first type (there's joke material in there for sure). It may be a relief to know that we don't come from bugs and slugs, but maybe not, depending on your comfort level with starfish. Yes, ancient ancestors of starfish, through zillions of cycles of natural selection, are believed to be the root of fish-like creatures that lacked bones, and that were the origin of bony fish, the primordial vertebrates, from which other vertebrates, including humans, sprang.
Body parts change during evolution to help organisms cope with their environment in new ways. There are certain things that have to be accomplished in order to survive. For example, you have to be able to meet nutritional demands, keep your fluids up to date, and defend against danger. And for your species to survive you have to reproduce. This list probably applies to all organisms, and, to some extent, even to simple single cell creatures like bacteria.
Each bacterium is responsible for its own survival. It has to take in sustenance from its surroundings to keep its internal machinery going. When it encounters something useful it wiggles its flagella to approach, but if the substance is toxic, it retreats. For example, if acid is squirted into a particular part of a petri dish, bacteria in the dish will move away, crowding up at the other end, much as people in swimming pool would put distance between themselves and a noxious chemical poured into the water. The same kind of story can be told for all animals. Through the long course of evolution, during which single cells combined to form multicellular life forms, and these combined to form complex organisms with multiple systems (for example, digestive, respiratory, reproductive, and nervous systems), more sophisticated solutions to these problems arose. But at a very basic level the fundamental problems faced by living things are fairly universal.
For vertebrates, the brain mechanisms that help solve these problems are remarkably similar. Unlike invertebrates, which are a diverse group with a variety of types of nervous systems, the brains of trout, newts, lizards snakes, pigeons, rats, cats, dogs, monkeys, apes, and people, to name just a few examples, all adhere to a common scheme. The hindbrain is necessary for life (damage there stops you from breathing), the midbrain for basic reflexes (like orienting towards a sudden sound), and the forebrain for all the stuff we think of as complex behavior and thought. Not surprising is the fact that the forebrain differs the most between mammals and other vertebrates, and the hindbrain the least.
Particularly relevant in the present context are the highly conserved systems within the mammalian forebrain that regulate behaviors involved in feeding, drinking, defense, sex and other functions necessary for the survival of the individual and species. There are of course differences in size and complexity in brain areas across mammals, but these differences are mostly variations on a common theme.
One important violation of the mammalian status quo is the human neocortex, and especially the prefrontal cortex, which does allow us to have some fairly sophisticated cognitive capacities. This justifies the common assumption that we are unique in our ability to think, reason, imagine, and create. But when we creatively imagine we are somehow special, standing on the top rung of a ladder of nature or near the heavenly end of a chain, we ignore the complexity of the natural world, and our place in it. We are part of a process, not its goal or final state. Just a branch point, a distal twig, on a continuously branching limb of the tree of life. Some may feel this perspective diminishes us. I don't. On the contrary, it helps me understand an important part of what it means to be human.