Our garden (rocky planet in the habitable zone where liquid water could exist) forms 4.54 billion years ago.
Prokaryotes (bacteria and other single-celled organisms lacking a nucleus) appear about 4 billion years ago.
Eukaryotes (organisms consisting of cells with a nucleus inside an envelope) appear about 2.5 billion years ago.
Complex lifeforms (such as bilaterally symmetric vertebrates with a circulatory system) emerges during the Cambrian explosion 541 million years ago.
Our solar biosphere continues to mature through cycles of extinction and proliferation over the next half a billion years.
When the time for reproduction approaches, Gaia releases a volatile catalytic biofilm (complex, tool-using species), which disperses widely (through proliferation, migration, colonization, and globalization). Its purpose is to accelerate the antientropic processes required for this stage of life, which it does by consuming most of the available planetary resources.
The completion of this task is preceded by an abrupt change in the composition of the atmosphere and the release of electromagnetic signals into space. Following this, seed pods are scattered in three volleys, one into local orbit (i.e. the first satellites), one across the solar system (Voyager et al.), and finally and most significantly one into nearby systems (exploration with the first warp drive).
At this point, the biosphere, having spent itself in the effort of seeding another garden, reaches the natural end of its life. With any luck, one of the seed pods has reached another potentially habitable planet. Cut off from all support, the droplets of residual catalyst (consisting of astronauts and pioneers) on the pod quickly evaporates, but not before introducing a hitchhiking microbe or two into the virgin sterile environment, beginning the cycle anew.
Ironically, the majesty of this cosmic circle of life is lost on the space travelers, who from their short-term, cerebrocentric perspective only see that their attempt at colonization has ended tragically, with the extinction of life on earth and the demise of the last humans on an unpromising dead rock. Cue the Lion King...
