Migration of brine shrimp causes substantial mixing of the water column. Migration of brine shrimp causes substantial mixing of the water column.

Lacking a centralized nervous system, coelom, anus and reproductive organs, the deep-sea flatworm Xenoturbella presents problems when it comes to its classification and teasing out its evolutionary history. Despite its simplicity, some of Xenoturbella's features appear to align it among the deuterostomes, the group of animals that includes ourselves. If true, this implies either a radical simplification of the body plan or the acquisition of many key deuterostome features independently by the various deuterostome groups. Two papers in this issue tackle different aspects of Xenoturbella, but together, move the field on a notch. Greg Rouse et al. add four new deep-sea species of Xenoturbella from the eastern Pacific Ocean to the two already known from the Atlantic. Phylogenomic analysis aligns them at the base of the Protostomia or even as basal bilaterians — much as would be expected from their simple morphology and not invoking radical simplification. Andreas Hejnol and colleagues come to a broadly similar conclusion based on robust phylogenetic analysis using eleven transcriptomes of Xeonturbella and acoel worms.

Sometimes it is the most unassuming animals that cause the most consternation. Xenoturbella (pictured) are simple marine flatworms with no brain, anus, gonads, excretory system or through gut, so one would expect them to find a home among the acoels — similarly simple animals thought to lie at the base of the evolutionary tree of Bilateri… Download references

The idea of a missing link between humanity and our ancestors predates evolution and popular science and actually has religious roots in the deist concept of the Great Chain of Being. Yet the metaphor has lodged itself in the contemporary imagination, and new fossil discoveries are often hailed in headlines as revealing the elusive transitional step, the moment when we stopped being animal and started being human. In The Accidental Species, Henry Gee, longtime paleontology editor at Nature, takes aim at this misleading notion, arguing that it reflects a profound misunderstanding of how evolution works and, when applied to the evolution of our own species, supports mistaken ideas about our own place in the universe. Gee presents a robust and stark challenge to our tendency to see ourselves as the acme of creation. Human exceptionalism, Gee argues, is an error that can infect scientific thought. Touring the many features of human beings that have recurrently been used to distinguish us from the rest of the world, Gee shows that our evolutionary outcome is one possibility among many, one that owes more to chance than to an organized progression to supremacy. He starts with bipedality, which he shows could have arisen entirely by accident, as a by-product of sexual selection, moves on to technology, large brain size, intelligence, language, and, finally, sentience. He reveals each of these attributes to be alive and well throughout the world-they are not, indeed, unique to our species. The Accidental Species combines Gee's firsthand experience on the editorial side of many incredible paleontological findings with healthy skepticism and humor to create a book that aims to overturn popular thinking on human evolution-the key is not what's missing, but how we're linked.

As the wild blue yonder beckons and labs and classrooms empty, Nature's regular reviewers share their holiday reads.

This paper discusses the too-big-to-fail problem from the perspective of individual banks and the financial system as a whole. It starts by exploring the potential implications of recent financial deepening and concentration, which has generated escalating expectations of state support, thereby encouraging further expansion and concentration. The paper then explores three policy approaches to tackling the too-big-to-fail problem. The first is the imposition of systemic surcharges of additional capital, which have the effect of reducing expected system-wide losses in systemically important banks, but not materially so at current levels of the surcharge. Second, new resolution regimes are being put in place to allow banks to fail safely — though the market still has doubts about their credibility for the biggest banks. Finally, structural reform of banks is taking place, through proposals by Volcker, Vickers and Liikanen. Despite this policy progress, expectations of state support remain high. This paper proposes potential additional reforms to tackle too-big-to-fail, such as placing limits on bank size and market share and increasing competition. While existing initiatives are a step in the right direction, there may be some distance to travel before banking is the right size.

Henry Gee relishes the memoir of Svante Pääbo, a leader in the field of ancient DNA.

The discovery, in 500-million-year-old rocks, of fossil acorn worms that lived in tubes illuminates the debate about whether the ancestor of vertebrates was a mobile worm-like animal or a sessile colony dweller. See Letter p.503

The vertebrate brain is so complex that tracing its origins among invertebrates is problematic. It must have evolutionary roots somewhere, but where? On page 289 of this issue, Pani et al. point the way with their finding that acorn worms have gene-expression signatures very similar to those that direct vertebrate brain development (A. M. Pani et al. Nature 483, 289–294; 2012). The closest relatives of vertebrates are the urochordates (tunicates, or sea squirts) and cephalochordates (the superficially fish-like lancelet). More distant on the evolutionary tree lie the hemichordates, which include acorn worms such as Saccoglossus kowalevskii (pictured), and the echinoderms (including starfish and sea urchins). Together, these groups comprise the deuterostomes, among which the elaborate structural and genomic innovations of vertebrates stand out rather like a cuckoo chick in a nest of willow warblers. One such feature is the signalling centres — physical regions of a developing embryo in which a specific set of secreted proteins drives the formation of a particular body section. These centres are present in the vertebrate neuroectoderm, an embryonic tissue that gives rise to the brain and nervous system. They are modified or completely absent in tunicates and lancelets, suggesting that many aspects of brain development are unique to vertebrates and arose de novo in the vertebrate lineage. Pani and colleagues' data contradict this theory. They show that a gene-expression program similar to that in three vertebrate neuroectodermal signalling centres is also present in S. kowalevskii. Although acorn worms do not have anything like a brain, Pani et al. show that these genetic programs are used to pattern (direct the development of) the ectoderm of the embryonic animal. The genes expressed are equivalent to those found in vertebrates, and they are deployed at similar times and places in vertebrate and acorn-worm embryos. The researchers propose that these signalling centres were part of an ancient gene-regulation 'scaffold' that was present in the common ancestor of all deuterostomes, and that was retained in hemichordates and vertebrates but lost in the evolutionary branches that formed lancelets and urochordates. Such an idea implies that the common ancestor was unexpectedly complicated. This does not mean that it had a complex brain, but that the genetic programs that were eventually modified for use in patterning the vertebrate brain already existed in an ancestral creature that lived perhaps more than 600 million years ago. Subscribe to comments

The artifice of the human world is something we take for granted. The bowl into which you pour your cornflakes of a morning, the spoon, the cup whence you sup your coffee, even the precise size and shape of the carton of milk, right down to the tiniest details of the replaceable cap, all have been designed. If one can never fail to be startled by such a revelation, then the designers of our world have done their job well. No more is this true than for art. Not necessarily the fine art that advertises itself as such (for if such art has its purposes, they can usually be discerned), but the graphic art that surrounds us; art that seems, on its surface, to be reportage, a portrait of reality. What is more, the closer to reality it appears, the greater the artist's skill not as reporter, but as illusionist.