Cactus ancestors are believed to have been members of the Caryophyllales order, specifically the family Cactaceae. This family is part of a larger group called core Caryophyllales which includes other succulent plants like carnations, beets, and spinach.

While the exact lineage is still being investigated, there is strong evidence that cacti evolved from non-succulent ancestors that lived in moist, tropical environments.

Here's a breakdown of what scientists have discovered:

* Ancient Relatives: The closest relatives of cacti are thought to be pereskioideae, a subfamily of cacti that still retains leaves and a more traditional woody stem.

* Adaptive Evolution: Over time, these ancestors adapted to drier conditions by developing succulent stems for water storage, spines for protection, and a reduced surface area to minimize water loss.

* Fossil Evidence: Fossils of cacti are relatively rare due to their fragile nature, but there are some important finds. The oldest known fossil cactus, *Pereskia globosa*, dates back about 50 million years.

While the exact evolutionary path is still being pieced together, it's clear that cacti have undergone a remarkable transformation, developing unique adaptations to thrive in some of the harshest environments on Earth.