  The atomic nucleus is made of protons and neutrons (nucleons), that are
themselves composed of quarks and gluons. Understanding how the quark-gluon
structure of a nucleon bound in an atomic nucleus is modified by the
surrounding nucleons is an outstanding challenge. Although evidence for such
modification, known as the EMC effect, was first observed over 35 years ago,
there is still no generally accepted explanation of its cause. Recent
observations suggest that the EMC effect is related to close-proximity Short
Range Correlated (SRC) nucleon pairs in nuclei. Here we report the first
simultaneous, high-precision, measurements of the EMC effect and SRC
abundances. We show that the EMC data can be explained by a universal
modification of the structure of nucleons in neutron-proton (np) SRC pairs and
present the first data-driven extraction of this universal modification
function. This implies that, in heavier nuclei with many more neutrons than
protons, each proton is more likely than each neutron to belong to an SRC pair
and hence to have its quark structure distorted.
