From plants and bacteria to animals and humans, our world is filled with many living things. In fact, there are approximately 8.7 million species of plants and animals in existence, and only 1.2 ...

Background The evolutionary origin of the eukaryotic nucleus is obscure and controversial. Currently preferred are autogenic concepts; ideas of a symbiotic origin are mostly discarded and forgotten.

Cells are the smallest unit of life. Cells in multicellular plants and animals are arranged into tissues, organs and organ systems. Cells. Tissues. Organs. Organ systems. And the organism itself.