Relationships between orders of Mammalia are difficult to determine because the respective family trees diverged during a relatively short time at the end of the Cretaceous Period just over 65 million years ago. The consequence of this has been that analyses hitherto have yielded contradictory results: some led to primates being grouped with rodents, as traditional morphological methods have done, while others seemed to show that primates were closer to carnivores.

The advantage: data from entire genomes

Since the complete genetic data for humans, mice and dogs have become available in the meantime, Gonnet and his colleagues were able to base their analysis on more information than has been available to any previous study. To eliminate possible systematic errors of individual methods, the researchers used more than ten different family tree reconstruction methods. In addition, analyses included genetic data from rats, chimpanzees, macaques and cows. The scientists used a marsupial, the opossum, as an outgroup to “anchor” the mammalian family tree to a root. Although not all the methods used yielded the same relationship distances, the topology of the family tree remained identical.

On the scent of the relationship

This finding contradicts the theory favoured until now, namely, grouping primates with rodents. According to the ETH scientists, in subsequent analyses there is a need to take yet more genetic data from representatives of the various orders into account in order to obtain additional evidence that primates and carnivores have more recent common ancestors than those they share with rodents. However, if the relationship between primates and carnivores becomes more firmly established, it will also shed new light on the choice of suitable model organisms.

Various possible relationships between humans, dogs and mice. ETH researchers found evidence proving that humans and dogs are closest to one another (circled in red). The Opossum was used as the outgroup. (Figure: PLoS Computational Biology) large

Finally, the closeness of the relationship between humans and dogs also received support from another scientific discipline not aimed at family tree analysis. Israeli and American researchers discovered that dogs and undergraduates follow odour trails in a similar way (3). When test persons wearing blindfolds and earplugs were asked to follow a chocolate odour trail, they chose a pathway whose shape was similar to that of a dog tracking the scent of a pheasant.