It's one of the most critical events inthe development of life on Earth. But we may have been thinking about it allwrong.
Our planet would be vastly different had eukaryotic cells never evolved
Take a walk in the woods and what do yousee? Trees reaching skywards with birds in their branches and, attheir roots, mushrooms pushing through the leaf litter. These, and all theorganisms you can see with the naked eye, have one fundamental similarity. Likeus, they are constructed from the same kind of cell. Under the microscope, thedifferences between plants, animals and fungi fall away to reveal a commoninternal structure.
The biosphere would be unimaginablydifferent had this “eukaryotic” cellnever evolved, making its origin one of the most critical events in thedevelopment of life on Earth. Almost everybody agrees that the complexeukaryotic cell evolved from a simple ancestor. The question is how.
Inside the eukaryotic cell is an intricatemeshwork of membranes called the endoplasmicreticulum (ER), interspersed with other structures suchas the energy-generating mitochondria. At the core is thenucleus, 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 theplanet would be simple cells known as prokaryotes – bacteria, for example – which under amicroscope appear as little more than tiny gel-filled sacs.
Biologists have always assumed that theeukaryotic cell evolved when a prokaryote folded parts of its outer membraneinwards, pinching off portions to generate internal compartments (seediagram). Some membranes, it is imagined, wrapped around the DNA to make themembrane of the nucleus, while others morphed into the ER. And atsome time during this process, free-living bacteria are thought to have been engulfedby the rest of the cell in a process akin to swallowing, calledphagocytosis. These bacteria went on to become mitochondria. While lots ofvariants of this model have been developed over the years, all make theimplicit assumption that eukaryotes evolved from the outside in – by pulling pieces ofexternal membrane and mitochondria into the cell.
看完单词再来阅读是不是觉得简单很多呢？
大家一定要记住：即使不做题，也要保持阅读英文材料的习惯哦，坚持一段时间，你就会发现你的阅读速度大大提高了呢！
