Here’s a list of the “givens” that we have observed:
• All living things are made out of cells, which are made out of a few kinds of naturally-occurring chemical elements.
• All cells have cell walls made out of a fatty substance called lipid bilayers.
• All cells have a “code” in the form of DNA. The pattern of chemicals in DNA react to form proteins, which make up most of an organism’s “body.”
• When a cell reproduces, the new cell gets a copy of the DNA.
• Sometimes, the DNA copies incorrectly in a few places. This means that the new organism’s DNA will form proteins in a different way than the old organism. This means the new organism will have a different “body” than the old organism.
• Some “bodies” are better at surviving in an environment than others.
• Those bodies will reproduce their DNA more than other bodies.
• Some cells form groups that seem to work together, such as slime molds.
• The cells in such a group are better able to survive if they specialize in certain tasks. For example, in slime molds, only a few cells in the group reproduce. Others grow stalks to support the reproducing cells, so their spores can fly further away.
• Thus, a “group” of cells can resemble a single body. Another example is the sponge. What is a sponge? It’s basically a group of single cells. But the cells work together closely enough that some scientists call the sponge an “animal,” a single body. You can put a sponge through a fine-mesh strainer and all the individual cells will come back together.
• Multicellular plants and animals are still made of groups of cells, which contain identical DNA, which codes for the proteins that make up the plant and animal body. Changes to the DNA of a plant or animal’s DNA causes changes in the bodies of plants and animals.
• Some animals and plants reproduce similar to how cells reproduce. They “bud off” new bodies. Some jellyfish and corals do this.
• Many animals and plants use sexual reproduction. Just like the slime mold, only a few cells in the body are specialized to reproduce. Sexual reproduction—unlike the other kind—also “mixes up” the DNA of the new organism. This creates more variety in animal and plant bodies.
• Some animals have hard parts in their body, like bones or shells, made out of absorbed minerals. On rare occasions, these hard parts form fossils.
• Scientists have dug up all kinds of fossils. In general, you can lay out fossils from deeper layers of rock next to fossils from shallower layers and compare them.
• If you compare the fossils, in general, they form a sort of trajectory or direction, where it’s easy to see how new “body plans” come from older ones.
• Thanks to modern genetics, we can compare the DNA of living species directly.
• These comparisons generally line up with the comparisons we’ve made from the fossils.
• We have also observed changes in various plant and animal DNA leading to changing body plans over time, causing so great changes that the new plant/animal no longer mates with the old plant/animal. This is called “speciation.”
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That’s an extremely, extremely, simplified list of many of the things we know and have observed about the natural world.
Evolution just ties them all together. It says that different organisms come about through mutation (changes in DNA) and natural selection (certain body plans surviving and reproducing more than others).
