It's a commonly held belief that we as humans are very curious about ourselves. We want to know why we exist. We want to know how we got here. We want to know what's going to happen to us when we die. However, for all of this curiosity, it seems to me that many of the attempts to answer these questions are made from a narcissistic context. The one I want to talk about is the question of What are we? Throughout school, this question, at least from a biological standpoint, seems to me to be stuck in the "everything must revolve around the earth" mindset. We learn about plants and how they differ from us in that they breathe carbon dioxide instead of oxygen. We learn about fish and how they breathe using gills instead of lungs. I'm remebering everything I learned outside of humans as being how these other things are different from us. As far as learning about things intrinsic to humans, that again was half blind in that when learning about how our four chambered heart worked, we never did so in the context of how other creatures circulatory systems functioned. So I've been reading through the Tree of Life website for a few hours now and am seeing it all, finally in a context, one step further away from ourselves than I had been.
So what are we? Assuming we're talking about matter, the first which makes us unique from other things is that we're reproducing machines. This seperates life from non-life. We are related to our ancestors because our makeup is based on the instructions that manifested their makeup. Rocks are similar to other rocks because the process that creates them is the same, not because any rock causes another rock to be like it.
Within the arena of life, what's the first thing that sets us apart. I'm speaking in a heirarchtical sense, from the most broad to the most specific. Firstly its the complexety of our cells. Specifically that our cells have a nucleus. We're Eukaryotes. Why? Because our cells have organelles and a nucleus unlike Prokaryotes.
Next, we're Animals. Some of the things which set us apart from plants, fungi and protists are that our cells lack the distincitve cell wall of plants and fungi, our cells are not photosynthetic (we can't turn sunlight into energy), and we are motile (we can move). There are a number of semi overlapping characterstics that set us apart at this point.
Next, we're bilateral and our cells have three germ layers. It usually doesn't occur to me but we're symetricaly bilateral and lots of other things aren't. We're like half a body standing up against a mirror. Plants aren't like that. Fungi aren't. It's so pervasive to think of myself and things like me (other animals) as being symetrically shaped that I don't even realize it. Having three germ layers has to do with the makeup of our basic tissues when we are embryos. Jellyfish (Cnidarians) have two germ layers.
Next, we are Deuterostomia. Here is a great description of what this means from the UC Berkeley Museum of Paleantology :
The name deuterostome means "mouth second", and refers to one important developmental feature unique to this group. To understand this feature, a little animal embryology is necessary. In the earliest stages of embryo development, when there are only a few cells and the embryo resembles a tiny globe of cells, a small pucker develops on one side of the embryo. This grows into a pocket, and allows some cells to migrate inside to form an additional layer of cells within the outer layer. At this stage, the embryo is known as a gastrula. In the Protostomia, which is the other major group of the Bilateria, the mouth develops from the edge of this pocket, where the inner and outer layer of cells meet; the anal opening develops later. In the Deuterostomia, the reverse is true; the pocket edge develops into the anus, and the mouth is formed later. Hence the byline at the top of this page: "Your mouth comes second."
This sets us apart from worms, molluscs, and molting animals (arthropods) among others.
Next, we're chordates. Chordates have a few distinct characteristics. I think the pharyngeal gill clefts and arches are the most interesting in that for different taxonomic subgroups the gills and arches turn into different things.