What are some factors that have caused the dominant life-forms on Earth to
change over time?

1) Mass extinctions
-every time Earth has had a mass extinction, it is followed by a rapid evolution of new species
-we lose a lot of species but we also have opportunities for new species
2) Continental drift
-continents don't stay put, move around earth's surface
-when it moves, the climate changes
-the species will have to adapt to the new climate
-best example: Antarctica used to be at the equator

That is true tge cactus as a structural adaptation to its envirinment has it leaves reduced to spines. but this doesnt mean it cant produce food it produces food because of the presence of chlorophyll in its stem