“Nature is the source of all true knowledge. She has her own logic, her own laws; she has no effect without cause nor invention without necessity.” ~ Italian polymath Leonardo da Vinci
2.5 billion years ago, after the continents had stabilized, prokaryotic sociality provoked a major evolutionary event: an archaean host and endosymbiotic bacterium committed to everlasting partnership through irreversible specialization; thus arose single-celled eukaryotes.
The concept caught on. Within a few hundred million years, a variety of eukaryotic cell types had arisen, including plants, fungi, and metazoa: the precursor to animals. The biggest difference in lifestyle among them was diet.
It seems that Nature has taken pleasure in varying the same mechanism in an infinity of different ways. ~ French philosopher Denis Diderot in 1753
With quantum exactitude, plants evolved the wondrous ability to turn sunlight and water into energy, and so were autotrophic. Though heterotrophic, fungi were flexible in eating ready-made foodstuffs from other organisms, dead or alive. In contrast, proto-metazoa were particular: requiring a diet of fresh, prefabricated amino acids and vitamins.
Many unicellular organisms have a colonial form: congregating when food becomes scarce, or as a defense. Single-celled green algae exposed to unicellular predators are easy prey. Clumping makes a difference. Small cell colonies offer an ideal trade-off between security from predation and maintaining sufficient surface area for nutrient uptake.
The next evolutionary step from aggregation was labor specialization at the cellular level. Prokaryotic colonial cells are not differentiated, but cells coordinate to perform different tasks.
Multicellularity was a déjà vu to the emergence of eukaryotes. Both were spurred by cellular cooperation.
Whence animals arose. In their innocence, early worms had no idea what trouble their inheritors a billion years hence would bring to their beloved home planet. In evolutionary terms, humans would turn descent into a dirty word.
The evolution of multicellular eukaryotes was by no means a declaration of independence; quite the contrary. Eukaryotes have constant association with their prokaryotic forbearers in the form of a microbiome.
Animals are so dependent upon their microbial companions as to require their assistance in development, digestion of foodstuffs, and protection from pathogens. An animal’s microbiome is essential. In ignorant ingratitude, we call our most important friends germs.
“The human body and its symbionts can be viewed as a community of interacting cells.” ~ American microbiologist David Relman
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In understanding evolution, a microscopic point of view makes a meaningful point. Genetic elements do not compete, nor do cells. Instead, the consistent themes of evolution, beginning at the cellular level, are: adaptation, cooperation, coordination, and specialization.
Multicellular life arose by endosymbiosis: the realization of mutual advantage by union. Photosynthetic chloroplasts and mitochondria, which are the power plants in plant and animal cells, were the products of unification and specialization.
As life grew more complex, cells of all sorts were symbiotic add-ons. Greater complexity was achieved mostly through modularity. Once a structure is established, the potential exists for fractal repetition for different functionality.
“In modern complex organisms, novel adaptations result mostly from reorganization of existing structures.” ~ Russian evolutionary biologist Alexander Badyaev
Organs are macroscopic extensions of the organelles within cells. The main innovation in plants was modularity: the evolution of semi-autonomous regions embodied within the embryonic meristems of roots and shoots.