Since all of this discussion is on extraterrestial life or intelligent Extraterrestrial life and Theory of Evolution, why not totally agree with it.
Why not this idea, as it would sum up everything.
About 700 Million years ago, an advance space traveling civilization existed. This species was exploring the Sol system, having arrived from the Alpha Centuari Binary System. As they needed supplies, water resources or some other chemicals, they landed on the second "Pale Blue Dot" from the Star. With the technological advance machines the aliens, took some water from 'Earth'. While walking around various mud puddles, one of the aliens 'spat' into the pool of water. The aliens, in their advance space craft, continued upon their journey into the vast beyond of space, eventually to become extinct and all traces of their technology, destroyed by geological process throughout the universe.
Panspermia--bacterial life, is thus true.
The rise of life, the PreCambrian Explosion, is thus true.
The Theory of Evolution, is thus true, as species radiate out into ecological niches and adapting to new habitat.
Eventually, a reptilian species, evolves with the potential of becoming Earth's first intelligent species. Unfortunately, an asteroid or secondary moonlet orbit is degrading and within some geological time will collide with the Earth, destroying the reptilian species and leaving fury mammalians to begin the new process of genetic changes.
Overtime, another species has the potential for intelligent developments--primates. This species, developes into H. sapiens neanderthalis and H. sapiens sapiens, of which the later wins the 'brainy' contest. Unfortunately, H. sapiens sapiens developed and relied upon technology, building atomic bombs and antibacterial soaps. The species, didn't develop into a Spacefaring Planetary Terraforming species. It didn't allocate resources to build secondary colonies on Mars, or attempt to terraform Venus--the cost basis was too high. Therefore, H. sapiens sapiens, continued to have political, ethical and religious debates within its society. The population continued to increase, natural resouces and environments were depleted. The population rose to 7.5 billion individuals, beyond the carrying capacity of 3.8 billion individuals of the Earth. The H. sapiens sapiens species sufferred a population crash, wiping it into extinction.

Dust to Dust, ashes to ashes...the little bacterium and microbe swim in a mud puddle!