The tree view of life

All of you will be familiar with those educational graphics that put into stark perspective how tiny is our place in the Universe, how on a clock that began ticking with the origin of life, humans evolved less than a minute before midnight. And the time since Current Biology began, 25 years ago, would be the merest fraction of a second. Yet, with our distorted perspective, we have felt this anniversary worth noting, as we did back in January with a special issue on ‘The Biology of Fun’ [1]. We took the opportunity then to note the fun we have in producing the journal and to demonstrate, through the collected articles, the importance of ‘fun’ as a biological concept. For our second special issue of this anniversary year, we thought we should take a broader view, and consider the history of life on Earth. The articles in this issue provide an eclectic look at that history, from ideas about the origin of life, through key developments such as the ‘great oxygenation event’ and mass extinctions, to the evolutionary history of particular phylogenetic groups. Those familiar with Current Biology will know that we have a long-running interest in the diversity of life on Earth and further directly relevant articles from our archive are collected on a special webpage (www.cell.com/currentbiology/history-of-life-on-earth). Much of the focus of modern biology is on delving in ever greater detail into the mechanisms by which living things work, with insights derived primarily from studies on a handful of ‘model species’. The general idea is that what is true of E. coli is true of an elephant, a unity demonstrated perhaps most strikingly by the highly conserved genetic code relating nucleotide and protein sequences in all cells [2]. But biology has a second dimension, with phylogenetic variety that has its own scientifi c value and fascination. That variety raises its own questions and sheds light on all biological issues, for example by highlighting the contingencies and constraints of evolution. Taking the broadest view of the evolution of life on Earth, it is clear that, from the formation of the earliest cells, considering life as a whole there has been elaboration, an increase in complexity. Yet it is also clear that evolution is not driven by ‘progress’ — with the exception of the artifi cial selection we have used in the domestication of animals and plants for our own purposes, evolution is a blind watchmaker, and can lead to loss and simplifi cation as well as gain and an increase in complexity. That evolution can involve loss is well illustrated by blind cave fi sh: as explained in a dispatch in this issue [3], eyes contribute a signifi cant proportion of the energy demands of an organism, and when they are not useful — as in the darkness of a cave —there is a strong pressure to save energy by losing them. There are many lineages of animals where there seem to have been signifi cant decreases in complexity, for example in the evolution of the parasite lifestyle, something which complicates attempts to understand evolutionary history of traits such as complex, centralized nervous systems. As living things have evolved and diversifi ed, fi rst in the oceans and then on earth, they have dramatically changed the environment they inhabit [4], something which in turn has created new opportunities for further evolution. Such changes have perhaps never occurred so rapidly as at present, due to the environmental depradations of humans — quite unique in our combination of individual size, population number and level of consumption. What this means for the future of life on Earth, on a long timescale, remains to be seen; somewhat less portentously and on a much shorter timescale, we look forward to the next 25 years of Current Biology. A big thank you to all the authors who have contributed articles to this special issue, and again (see [1], end) to my excellent colleagues at Current Biology and Cell Press generally — in particular to Florian Maderspacher, who played the biggest part in conceiving and implementing this issue.