How to Build a Human: Adventures in How We Are Made and Who We Are. Philip Ball

it could be from viruses. But another hypothesis has been suggested by evolutionary biologists Eugene Koonin and Bill Martin: the nucleus is there to slow down the process of protein production from the genome. Recall that the genome of eukaryotic cells (but not that of prokaryotes) is full of rogue bits of DNA called introns that interrupt the gene sequences encoding proteins. It’s thought that these introns might be the remnants of an infestation of so-called “jumping genes” – pieces of DNA that are adept at splicing themselves at random places in the genome. Many eukaryotic introns are ancient: they appear in the same places in equivalent genes in a variety of eukaryotic organisms ranging from humans to yeast. This suggests that there was an episode far in the evolutionary past when the genomes of eukaryotes became particularly vulnerable to infestation by jumping genes.¹

      Whatever the reason, introns now need to be cut out before proteins are made. This happens after the transcription of DNA into RNA, the intermediary molecules that serve as the templates for protein synthesis on the structure called the ribosome. An RNA transcript of a gene is made from DNA, and then it is edited by special enzymes before being used by the ribosome to guide protein synthesis.

      In bacteria, this sequence of transcription to ribosomal RNA followed by translation to proteins happens all at once; the RNA is translated even while it is being transcribed. If that happened in eukaryotes, there would be no time for proper intron editing. But the nucleus separates the process of transcription, which happens inside its membrane, from translation, which happens outside. Maybe, say Koonin and Martin, this spatial separation of transcription from translation ensures that the job is done properly.

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      Since eukaryotes are more complex than prokaryotes, it seems natural to suppose that the latter cells appeared first and eukaryotes evolved from them. This is indeed what is suggested by both the fossil record (even single-celled organisms leave fossils of a kind) and studies of DNA, from which we can deduce how the “tree” of evolution branched.² However, the differences between prokaryotes and eukaryotes are rather profound, and it’s not obvious how to get from one to the other along the gradual steps that evolution tends to take.

      It’s now believed that this isn’t how it happened. Rather, eukaryotes are thought to have appeared by the abrupt merging of simpler cells.

      The Earth is about 4.6 billion years old, and life seems to have begun at least by 3.8 billion years ago. It consisted of nothing but single-celled prokaryotes for perhaps as much as three billion years after that; the first multi-celled eukaryotes don’t appear in the fossil record until around 600 million years ago. No one knows what those first organisms were like, but it’s possible that they resembled the slug-like aggregates of Dicty, now permanently united into a single body. Alternatively, they might have been similar to some of today’s sponges.³