text: One possible solution, inherent to eukaryotic cells but not to bacteria, is sex. That there is a link between sex and complexity has been argued persuasively by Mark Ridley in Mendel’s Demon. The trouble with asexual reproduction, says Ridley, is that it is not good at eliminating copying errors and harmful mutations in genes. The larger the genome, the greater the probability of a catastrophic error. The recombination of genes in sexual reproduction may lower this risk of error, and so raise the number of genes an organism can tolerate before undergoing a mutational meltdown (although this has never been proved). Clearly, however, the more genes an organism accumulates, the greater its possible complexity, so the invention of sex in eukaryotes might have opened the gates to complexity. While there is almost certainly some truth in this argument, there are also problems with the idea that sex stands at the gateway to complexity, as Ridley himself concedes. In particular, the number of genes in bacteria is well below the theoretical asexual limit, even if they relied on asexual reproduction alone, which they do not (lateral gene transfer in bacteria helps restore genetic integrity). Ridley acknowledges that the data are ambivalent, and the asexual limit to gene number may fall somewhere between fruit flies and human beings – if so the gates of complexity could hardly have been thrown open by the evolution of sex. Something else must have been the gate-keeper.
  I do think there was an inherent tendency for eukaryotes to grow larger and more complex, but the reason relates to energy rather than sex. The efficiency of energy metabolism may have been the driving force behind the rampant ascent of eukaryotes to diversity and complexity. The same principles underpin energetic efficiency in all eukaryotic cells, giving an impetus to the evolution of larger size in both unicellular and multicellular organisms, whether plants, animals or fungi. Rather than being a random walk through vacant niches, or a march driven by the imperative of sex, the trajectory of eukaryotic evolution is better explained as an inherent tendency to become larger, with an immediate payback for an immediate advantage – the economy of scale. As animals become larger, their metabolic rate falls, giving them a lower cost of living.
  I am here conflating size with complexity. Even if it is true that greater size is favoured by a lower cost of living, is there really a connection between size and complexity? Complexity is not an easy term to define, and in attempting to do so we are inevitably biased towards ourselves: we tend to think of complex beings in terms of their intellect, behaviour, emotions, language, and so on, rather than, for example, a complex life-cycle, as in an insect with its drastic morphological transitions, from caterpillar to butterfly. In particular, I am not alone in my bias towards larger size: for most of us, I suspect, a tree appears more complex than a blade of grass, even though, in terms of photosynthetic machinery, grasses might be said to be more highly evolved. We insist that multicellular creatures are more complex than bacteria, even though the biochemistry of bacteria (as a group) is far more sophisticated than anything we eukaryotes can muster. We are even inclined to see patterns in the fossil record implying an evolutionary trend towards greater size (and presumably complexity), known as Cope’s Rule. While accepted with little question for a century, several systematic studies in the 1990s suggested that the trend is nought but an illusion: different species are equally likely to become smaller as they are larger. We are so mesmerized by our fellow large creatures that we easily overlook the smaller ones.