After 2 billion years of peaceful roaming through ancient seas filling the atmosphere with oxygen in the process, primitive bacteriae jumped to a new level of organization.
First, it was the probable association within a single cell of different organisms, in a symbiotic arrangement benefiting both.
It is thought the ubiquitous mitochondria, for example, is what is left of a once different organism that acquired the ability to harness oxygen for a more energetic metabolic cycle, while a host lacking that ability readily provided shelter in exchange for energy.
Mitochondria possess a small amount of genetic material to support this hypotheses; better yet, the genome is related to a probable typhus bacteria ancestor having the same ability to penetrate host cells as today.
Similarly, chloroplasts are probably what is left of cyanobacteria (blue-green algae) that learned to synthesize clorophile and harness light for photosynthesis. Animal and vegetal life then diverged by the way they integrated helper organisms.
By the same token, it is possible the cell nucleus proceeds from a similar association, in this case with an organism that had developed a reliable and efficient genetic material handling and copying ability; its genes then merged in a larger whole.
The transition from prokaryotes (non-nuclear) to eukaryotes (nucleated and more evolved) signaled the first far-reaching advance after the emergence of life itself.
The next and almost immediate step in the hierarchy, was the association of multiple individuals and specialization in function for a more successful organism. An example these days, though not fully related with what happened then, is the ubiquitous bacterial plaque, that responsible for cavities as well as fouling of steam pipes and other nuisances.
Plaques are an association of thousands, sometime millions of individual bacteriae in a disposition where some embryonic form of specialization has emerged, such as structural, or fluid circulation roles and so forth.
Shaped by disaster
We have reviewed in this topic some potential hazards lurking out there. They are potential in the sense of being unlikely within a human lifetime frame. They are absolute certainty in geologic time frames, and we have learned to unearth clues to a violent past.
Perhaps the most easily understood and publicized kind, is the collision of an asteroid or comet large enough to bear a significant imprint both from the blow, and as result of longer term climatic effects.
We are now fairly certain a kilometers-wide body plunging near Yucatan, caught dinosaurs ill-prepared to face the climatic consequences, spelling their doom.
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