ASome of the world's most famous museums are home to objects that did not originate in the countries where they are now displayed. The British Museum holds sculptures removed from the Parthenon in Athens; museums across Europe and North America contain bronzes taken from the kingdom of Benin, in present-day Nigeria, during a military raid in 1897. For much of the twentieth century, the presence of such objects in foreign collections attracted little comment. In recent years, however, campaigns for their return — a process known as repatriation — have grown louder, and the question of who should own the world's cultural treasures has become one of the most contested issues in the museum world.

BThose who defend keeping such objects often appeal to the idea of the "universal museum." On this view, the great encyclopedic museums perform a valuable service by gathering the achievements of many civilisations under one roof, where they can be studied together and seen, often free of charge, by millions of visitors from around the world. An object displayed in such a setting, the argument runs, belongs not merely to one nation but to all of humanity, and removing it would impoverish a shared inheritance.

CSome defenders go further, raising practical objections to wholesale return. If every object were sent back to its place of origin, they warn, the world's major museums would be stripped of much of their content, and a flood of competing claims might prove impossible to resolve. A few have argued that certain source countries lack the facilities to display and protect fragile artefacts — though this argument is now heard far less often, and is widely regarded as both outdated and condescending.

DAgainst all this stands a powerful moral case for return. Many of the most disputed objects were not bought or freely given but seized by force, looted during wars, or acquired under the unequal conditions of colonial rule. To the communities from which they came, such objects are frequently far more than works of art: they are sacred items, royal regalia or embodiments of cultural identity, whose absence is keenly felt. Campaigners argue that no claim about universal access can outweigh the injustice of holding on to property that was wrongfully taken in the first place.

EIn recent years, the balance of opinion has begun to shift. A number of institutions have agreed to return contested items: several museums in Germany and elsewhere have committed to handing back Benin bronzes to Nigeria, and other returns have followed. Technology has opened up new possibilities, since highly accurate digital scans and replicas allow an object to be studied, and even exhibited, in more than one place at once. Meanwhile, several countries that were once told they could not care for their heritage have built modern, world-class museums of their own.

FYet the issue is rarely as simple as it first appears. Not every object in a foreign museum was stolen; some were lawfully purchased or presented as gifts, and the circumstances of acquisition vary enormously from case to case. For this reason, many observers favour flexible solutions rather than blanket rules. Long-term loans, shared ownership, joint exhibitions and collaborative research projects all offer ways for institutions in different countries to work together, so that the benefits of access and the demands of justice need not always be in conflict.

GOn balance, however, the old confidence of the universal museum looks increasingly difficult to sustain. The claim that the world's treasures are best kept in a handful of Western capitals rests on assumptions that fewer and fewer people are willing to accept, and the moral weight of the case for returning objects taken unjustly is hard to deny. This does not mean that every item must be sent back tomorrow, or that museums have no role as guardians of world culture. It does mean that they would be wiser to engage openly with the communities making these claims than to defend the practices of the past.

AAmong the most thought-provoking patterns in the natural world is convergent evolution: the tendency for unrelated organisms, living in similar conditions, to evolve strikingly similar features quite independently of one another. The sleek, streamlined body shape of the shark, the dolphin and the extinct marine reptile known as the ichthyosaur is a famous example. These three animals are only distantly related, and their common ancestors looked nothing alike, yet each arrived at almost the same design in response to the demands of moving efficiently through water.

BExamples of convergence are not hard to find. The complex, image-forming eye, with a lens that focuses light onto a sensitive surface, evolved separately in vertebrates such as ourselves and in the octopus and its relatives, despite the two lineages having diverged hundreds of millions of years ago. Powered flight appeared independently in insects, birds, bats and the extinct pterosaurs. On the island continent of Australia, marsupial mammals evolved forms remarkably similar to the placental mammals found elsewhere, including a marsupial that strongly resembled the sabre-toothed cats of other continents.

CThe deeper significance of these patterns becomes clear in a famous thought experiment. Suppose it were possible to rewind the history of life to some distant point and let it run again from the beginning. Would the same kinds of creatures appear the second time around? One influential school of thought, associated with Professor Daniel Okafor, holds that they would not. According to this view, evolution is dominated by chance events — a chance mutation here, a mass extinction there — so that its outcomes are essentially unrepeatable. Rewind and replay the tape of life, Okafor argues, and the living world would look utterly different.

DAn opposing camp draws precisely the reverse conclusion from the evidence of convergence. Professor Helena Marsh contends that the repeated, independent appearance of the same forms is exactly what we should expect if natural selection tends to channel life toward a limited set of optimal solutions. On this view, the broad outlines of evolution are surprisingly predictable: if the tape were replayed, eyes, limbs, flight and perhaps even intelligence would be likely to evolve again, because the environment rewards the same designs over and over.

ERecent research has added an unexpected twist to the debate by revealing convergence at the most fundamental level of all — that of the genes themselves. In some cases, the same genetic changes have been found to have arisen independently in separate lineages. The geneticist Dr Rahul Menon points to evidence that bats and dolphins, which both use echolocation to navigate and hunt, show similar changes in some of the genes involved in hearing, despite having evolved this ability separately. For Menon, such molecular convergence is powerful support for the predictability of evolution.

FNot everyone is convinced. Professor Ingrid Sørensen cautions that the importance of convergence is easily overstated. Many supposed examples, she points out, are superficial: traits that look similar from a distance often turn out to differ considerably in their details when examined closely. More fundamentally, she warns of a bias in how the evidence is gathered. Biologists naturally notice and celebrate the striking cases in which evolution has repeated itself, while paying far less attention to the vast number of organisms whose features are unique and have never been duplicated.

GWhere, then, does the truth lie? Most biologists now suspect that both camps are partly right. The recurrence of certain forms shows that natural selection is a powerful force capable of producing the same answer many times over; the countless one-off products of evolutionary history show that chance and circumstance also leave a deep mark. As the astrobiologist Dr Sofia Cabrera observes, the answer bears directly on what we might expect to find elsewhere in the universe: if convergence is as powerful as some believe, then life on other worlds, should it exist, might resemble life on Earth more closely than we have any right to assume.