COPENHAGEN, DENMARK—According to a Nature News report, researchers Enrico Cappellini, Claire Koenig, Ioannis Pastramanis, and Palesa Madupe of the University of Copenhagen used mass spectrometry to analyze the enamel of four Paranthropus robustus teeth recovered from South Africa’s Swartkrans Cave. The scientists were able to map some 425 amino acids, which are the building blocks of proteins, in the two-million-year-old hominin teeth. One protein identified in two of the samples is produced by a gene on the Y chromosome, showing that these individuals were males. The small size of one of these teeth had previously led researchers to attribute the tooth to a female individual. A protein produced by the X chromosome was detected in the other two teeth, indicating that they belonged to females. The researchers then used the amino acid sequences to build a simple evolutionary tree showing that modern humans, Neanderthals, and Denisovans are more closely related to each other than they are to Paranthropus robustus. Although it is unclear in this early stage of ancient proteomics if protein analysis will help scientists understand the relationships between different species, knowing the sex of an individual will help researchers to account for sex-based differences in size within a particular species.  For more on Paranthropus robustus, go to "Consider the Craniums."