Your answer is A. gene flow
Stromatolite fossils are the oldest known fossils<span>, that formed 3.5 billion years ago when the environments of Earth were too hostile to support life as we know it today. They </span><span>are layered mounds, columns, and sheet-like sedimentary rocks</span><span>
Stromatolite fossils indicate that </span>life must have arisen quite a bit earlier, perhaps 3.9 billion years ago.
Temperature, rainfall, and geological features would be the major abiotic factors that would determine plant distribution.