The billion-year time span saw the evolution of the animal body. Every step of the way, body features were adaptations for particular environments. In some cases, we still use those features for their original purposes. Cells adopted mitochondria in order to breathe oxygen. Eukaryotic cells were much more energetic and versatile, allowing for cell specialization and sexual reproduction. As cell colonies became multicellular animals, they needed circulatory and digestive systems to nourish every cell. The nervous system evolved to coordinate activities throughout the body. Eyes and teeth were important innovations for hunting during the Cambrian Explosion. Jaws allowed for a much more varied diet. An internal skeleton provided structural integrity, especially for the move to land. Internal fertilization (sexual intercourse) was also a necessary adaptation to reproduce on dry land.
Some body features are souvenirs of ancestral environments from days gone by. Our cells themselves are still based on salt-water chemistry because cells originated in the ocean. Ancestral fish evolved paired fins and swim bladders for the ability to swim up off the ocean floor, and those features are still with us as limbs and lungs.
By 300 MYA, our ancestors were not only animals, but amniotes. They were land animals with a complete internal skeleton, a full set of organs and glands, a head with a face, tongue, and middle ears, a neck, and four limbs all the way down to hands and feet with five digits. Eggs were fertilized inside the female body, and embryos were protected with an amniotic sac. It’s safe to say that our similarities to those ancestral reptiles outweigh the differences.
Our ancestors evolved concomitantly with the rest of the biosphere, always in a two-way relationship with Earth’s active geology. As living things exhaled varying amounts of oxygen, methane, and carbon dioxide, they affected climate changes as radically as hothouse ages and planetary snowballs. The slow buildup of continental crust provided a growing habitat for life along the shorelines. Microbes, plants, fungi, and invertebrates all colonized the land before tetrapods. Some of those plants, the Carboniferous forests, formed coal deposits that literally fuel our economy today.
The body is not the only feature that has been evolving. The mind also evolved in gradual increments. The animals of 500 MYA were most likely advanced enough to have conscious minds. Animal behavior and social relations played important roles for survival and reproduction. The most successful behaviors got passed down to future generations, whether through genes or learning.
Some philosophical and religious systems downplay the importance of the body. The mind or the “spiritual realm” is revered as a higher form of existence. In truth, animal nature cannot be separated from physical form. The mind is a product of the brain. Evolutionary pressure is entirely driven by the corporeal needs to eat, survive, and mate. The male sex drive and the maternal instinct evolved to promote reproduction, ultimately for the survival of particular genes. There is no way around it. We are biological through and through.
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