《We’ve got the evolution of complex cells inside-out》 Part1/3

来源：New Scientist

It's one of the most critical events in the development of life on Earth. But we may have been thinking about it all wrong.

Our planet world be vastly different had eukaryotic cells never evolved

Take a walk in the woods and what do you see? Trees reaching skywards with birds in their branches and, at their roots, mushrooms pushing through the leaf litter. These, and all the organisms you can see with the naked eye, have one fundamental similarity. Like us, they are constructed from the same kind of cell. Under the microscope, the differences between plants, animals and fungi fall away to reveal a common internal structure.

The biosphere would be unimaginably different had this “eukaryotic” cell never evolved, making its origin one of the most critical events in the development of life on Earth. Almost everybody agrees that the complex eukaryotic cell evolved from a simple ancestor. The question is how.

Inside the eukaryotic cell is an intricate meshwork of membranes called the endoplasmic reticulum (ER), interspersed with other structures such as the energy-generating mitochondria. At the core is the nucleus, a large compartment with a double membrane, within which lies the cell’s genetic material. Take away this type of cellular organisation and the only thing left on the planet would be simple cells known as prokaryotes – bacteria, for example – which under a microscope appear as little more than tiny gel-filled sacs.

Biologists have always assumed that the eukaryotic cell evolved when a prokaryote folded parts of its outer membrane inwards, pinching off portions to generate internal compartments (see diagram). Some membranes, it is imagined, wrapped around the DNA to make the membrane of the nucleus, while others morphed into the ER. And at some time during this process, free-living bacteria are thought to have been engulfed by the rest of the cell in a process akin to swallowing, called phagocytosis. These bacteria went on to become mitochondria. While lots of variants of this model have been developed over the years, all make the implicit assumption that eukaryotes evolved from the outside in – by pulling pieces of external membrane and mitochondria into the cell.