For the past decade, scientists have thought that DNA sequences from Neanderthals were found only in non-African populations. Neanderthals, a human species that died out around 40,000 years ago, lived mainly in Europe and parts of Asia.

As Homo sapiens migrated from Africa into Europe and Asia some 70,000 years ago, the two species mingled. Roughly 2% of the genomes of modern European and Asian populations is inherited from Neanderthals. But sub-Saharan Africans, it was believed, possessed no Neanderthal DNA.

A new study shows this is not true. Sub-Saharan populations do carry Neanderthal DNA. This is likely to have come not directly from Neanderthals in Africa but from ancient Europeans migrating back into Africa over the past 20,000 years.

For years, scientists believed that our solar system formed through violent collisions between the original building blocks. Now, thanks to data from the New Horizons spacecraft, they are changing their minds. Launched in 2006, New Horizons has travelled to the edge of the solar system, to the Kuiper belt, which consists of millions of rocks, remnants from the formation of the planets. The spacecraft’s images of an object named Arrokoth, which is made up of two connected lobes, suggest the rocks fused slowly rather than violently, leading scientists to revise their views of how the planets formed.

This is how science works – by reshaping theories as knowledge accumulates. For some, such as those who would deny the theory of evolution, this reveals science to be a false guide. Certainly, scientific knowledge is provisional and changes as we know more. It is also cumulative: the more we know, the more certain we can be. Both aspects are to be valued. They reveal not the weakness of science but its greatest strength.