- 我们从达尔文的理论中了解到所有的生命都源于同一个单细胞，今天地球上无数的物种都是由此进化而来的。动物和植物进化为不同种群的转折点是什么？它是怎样发生的？

　　根据进化理论，地球上的每个现存或死亡的生物体都起源于同一个祖先。这意味着，所有的生命形式都是彼此相关的——换句话说，地球上的所有物种都是远房表兄妹。有时，这种相关性特别明显——猿和人类在外表上确实特别接近（比如，我们的骨架很明显是它们的变异）。这是因为我们最后的共同祖先距今只有几百万年（通过比较我们的DNA并估计计算随机变异造成这种差异所需的工夫就能得出这一结论），因此我们在很多方面仍有共同之处。但是，与我们的亲缘关系更少的生物是什么情况？比如，我们与植物有什么共同之处？为何目前彼此的差距如此之大？

　　实际上，动物和植物在细胞层面上有很多共同之处。我们都是真核细胞，也就是说，我们的染色体存在于细胞的一个单独部分——细胞核——当中。因此，我们与细菌（准确的名字是原核生物）差距遥远，后者的细胞缺少细胞核。植物和动物也能够进行真正的有性繁殖，触及雄性和雌性配子——精子和卵子（种子植物的精子在花粉粒中被大量繁殖和携带）——的聚变。动物和植物也在细胞中有特别的能量站——即线粒体——利用氧气从糖中获得能量。令人惊疑的是，这些线粒体曾是被早期单细胞真核生物俘获的自生细菌。植物和动物在细胞层面上的主要差别是，植物的一个祖先（也被称为单细胞生物）第二次进行了这种外展——它俘获了另一种可以在光合作用中利用光从二氧化碳和水中产生糖的细菌。这些被俘获的细菌被称为叶绿体。

　　此后，与无法进行光合作用的真核生物相比，进行光合作用的真核生物可能在进化过程中出现显著的不同，原因很简朴：彼此的生活需求不同。进行光合作用的真核生物需要光、水、二氧化碳和矿物，而无法进行光合作用的真核生物则需要以其他细胞为食。当动物和植物后代分别成为多细胞体时，这种关键性的差异更为显著。植物成为静态的光采集体，而动物成为更加有效的捕猎者。

　　因此，植物祖先俘获的能够成为叶绿体的物质是植物和动物差异显著的主要原因之一，但另一个更简朴的原因是我们的祖先实际上在很久以前就开始分化了。详细的分化工夫虽然难以确定，但我们最后的共同祖先很可能存在于至少15亿年以前。它是单细胞水生真核生物，并应当具备上述动物和植物细胞的共同特点。从那时起，动物和植物就开始了不同的进化之旅。然而，这是一种过于简朴的观点。过去，所有的真核生物都被视为不是植物就是动物（实际上，细菌在被划分为原核生物前也曾被视为植物），但从19世纪中期开始，人们承认存在更多的生命分界。一些人现在认为，仅在真核生物中就有超过20个差异显著的生物群，足以组成不同的生命分界。很多是单细胞的（被通俗地称为原生生物），但其他的已独立成为多细胞体，比如菌类、红藻和褐藻（绿藻是植物）。而另外一些仿佛具备多细胞性，比如令人惊疑的粘液菌（根据环境不同，很多粘液菌在单细胞和多细胞状态之间转换）。虽然这一新观点引出的详细分界数量难以确定，但我想，它更真实地显示出生物体的深度多样性以及地球上的生命问题通过进化加以解决的多种进程。

　　From Darwin’s theory we know that all the lives are coming from the same single cell, which evolved into all the millions of species on earth today. What’s the turning point for animal and plant to evolve into such different species groups? How did it happen?

　　According to the theory of evolution, every living thing on Earth, alive or dead, has descended from a single common ancestor. This means that all life forms are related – we are literally distant cousins. Sometimes, this relatedness is readily apparent – apes and humans are really rather similar in appearance (our skeletons, for instance, are clearly variations on a theme). This is because our last common ancestor lived only a few million years ago (one can work this out by comparing our DNA and estimating how much time must have elapsed for the differences to accumulate by random mutations), so we still have much in common. But what about our more distant relatives; what could we possibly have in common with plants, for instance, and how have we ended up looking so different?

　　Well, animals and plants actually have rather a lot in common at the level of our cells. We are both eukaryotes, which mean that our chromosomes are contained in a separate compartment in the cell – the nucleus. We are thus distinct from the bacteria (prokaryotes to give them their correct name), whose cells lack nuclei. Plants and animals can also undergo true sexual reproduction, involving the fusion of male and female gametes: sperm and eggs (the sperm of seed plants is highly reduced and carried in pollen grains). Animals and plants also have special power stations in the cells called mitochondria, which use oxygen to get energy from sugar. Amazingly, these mitochondria were once free-living bacteria that were captured by an early single-celled eukaryote. The main difference between plants and animals at the cellular level is that an ancestor of plants, also single-celled, carried out this abduction a second time – it captured another kind of bacteria that could produce sugar from carbon dioxide and water using light, in a process called photosynthesis. These enslaved bacteria are called chloroplasts.

　　From this point on, those eukaryotes that could photosynthesise would be shaped by evolution in a very different way from those that couldn’t, simply because they needed different things to live. Photosynthesisers need light, water, carbon dioxide and minerals, whereas the others need to eat other cells. This key difference became very apparent when the animal and plant lineages independently became multicellular. Plants became static light-harvesters; animals became ever more effective hunters.

　　So the capture of what would become the chloroplast by a plant ancestor is one of the key reasons why plants and animals are now so different, but another, simpler reason is that our lineages diverged a very long time ago indeed. Exactly when is difficult to pinpoint, but it is likely that our last common ancestor lived at least 1.5 billion years ago. This ancestor was a single-celled aquatic eukaryote, and would have had the common features of animal and plant cells described above. Since then, our two great kingdoms have trod separate evolutionary paths. This is, however, a rather simplistic view. Once, all eukaryotes were viewed as either plants or animals (indeed, even bacteria were once treated as plants before being separated as the prokaryotes), but since the mid-nineteenth century the number of recognised kingdoms of life has grown. Some people now think that among the eukaryotes alone there are over 20 groups of living things that are different enough to warrant the status of kingdom. Many are single-celled (these are informally referred to as protists), but others have independently become multicellular, such as fungi, red seaweeds and brown seaweeds (green seaweeds are plants), and some seem to be experimenting with multicellularity, such as the fabulous slime moulds, many of which alternate between single-celled and multi-celled states depending on the environment. While the sheer number of kingdoms in this new world view is difficult to grasp, I think it gives a far more honest picture of the true deep diversity of living things; of the many ways in which evolution has solved the problems of life on Earth.(本文来源：网易探索 )

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