Humans and their closest primate relatives diverged evolutionarily some 6 to 8 million years ago. Since then, a wide range of traits and behaviors have come to distinguish our species from the rest of the genus Pan. The question of what, at a molecular level, produced those distinctions has long remained one of biology’s more stubborn puzzles. Scientists have pointed for decades to gene expression as the likely culprit, but pinning down the precise mechanisms has proven elusive.
That may be starting to change. Researchers from Stanford University and the Weizmann Institute of Science in Israel have published a study in the journal eLife that zeroes in on a process called DNA methylation, a chemical modification that, as described by the National Human Genome Research Institute, involves attaching methyl groups to specific locations within DNA, where they turn a gene on or off, thereby regulating the production of proteins that the gene encodes. Their findings reframe our understanding of what, biologically speaking, made us human.
A Hybrid Cell Experiment That Sounds Like Science Fiction
To investigate where human and chimpanzee biology diverges at the epigenetic level, the research team designed an experiment that raises eyebrows before it raises answers. Scientists fused together human and chimpanzee stem cells not to engineer a hybrid organism, which researchers have already attempted with problematic results, but to study the relationship between two types of regulatory mechanisms: cis-acting factors, which regulate genes on the same DNA molecule, and trans-acting factors, which regulate expression on target genes located in different DNA molecules.
DNA methylation profiles across diverse human, chimpanzee, and hybrid cell types – © eLife
The logic behind the design is subtle but important. By placing both sets of genetic material inside an identical cellular environment, the team could more cleanly separate methylation differences caused by local DNA sequence effects from those driven by broader, diffusible cellular factors. From those fused cells, the scientists grew hybrid neurons, liver cells, and muscle tissue, giving them a rare window into human-specific epigenetic patterns across multiple tissue types.
CpG Sites, Mutations, and a Ripple Effect Across the Genome
According to the study published in eLife, what the researchers found was that cis-regulatory mechanisms were the dominant drivers of methylation differences across the genome. At the center of their findings are structures called CpG sites, spots in the genome where a cytosine nucleotide sits directly adjacent to a guanine. These sites serve as key targets for methylation, which helps silence genes.
Contribution of cis and trans regulation to DNA methylation divergence between human and chimpanzee – © eLife
Single-letter mutations in the DNA can either create new CpG sites or destroy existing ones. When a site is lost, the chemical tag disappears with it; when a new one forms, a fresh location for silencing opens up. But the consequences do not stay local. The study found that these changes ripple outward, altering methylation patterns at neighboring CpG sites up to 50 base pairs away. Over evolutionary time, this cascade of small, sequence-level mutations has produced distinct epigenetic landscapes in humans versus chimpanzees.
From Epigenetic Shifts to Distinctly Human Traits
Those differences in methylation, it turns out, are not abstract. According to the researchers, the coordinated, lineage-specific shifts they observed appear to have shaped a surprisingly broad set of human-specific characteristics. Genes involved in cognition and synaptic plasticity in brain cells showed signs of these shifts, as did genes linked to delayed growth patterns during development, craniofacial and dental features, and even heightened susceptibility to hepatitis C infection.
“While gene expression divergence has long been considered the primary driver of human evolution, identifying the molecular mechanisms underlying uniquely human traits remains challenging,” the authors write. Their study makes the case that DNA methylation plays a coordinating role that had not been fully explored before this work.
As the authors put it: “We demonstrate that while both cis- and trans-regulatory mechanisms shape interspecies methylation differences, cis-acting factors predominate, thus directly linking genomic sequence variation and epigenetic divergence.” In their framing, the DNA sequence itself, not just the cellular environment, is what steers the epigenetic differences that set our species apart.