Asingle celled organism can be classified as a prokaryote based on the absence of

Nucleus

Explanation:

Prokaryotes are the singled cell organisms that do not have a true nucleus which means like eukaryotes they lack a membrane-bound nucleus. So the genetic material of prokaryote are present in the cytoplasm of their cell.

Apart from the nucleus prokaryotes also lack membrane-bound organelle like mitochondria or chloroplast which are present in eukaryotes and helps in ATP formation for the cell. So prokaryotes use the plasma membrane of cell for production of ATP.

Therefore an organism can be classified as a prokaryote if it lacks a nucleus and other membrane-bound organelles.

answer: land bridge theory. land bridge theory held that shifting portions of dry land explained the finding of identical species on different continents. plate tectonic theory eventually eliminated the land bridge theory.