Ancient DNA has changed the human family tree by showing that modern people carry genetic material inherited from Neanderthals and Denisovans. Yet that remarkable record is incomplete. DNA survives only under favourable conditions, and many populations that once existed may never be represented by a sequenced bone. A new method called TRACE tries to recover their presence from the genomes of living people instead.
TRACE analyses ancestral recombination graphs, statistical reconstructions of how segments of modern genomes are related through time. When a stretch of DNA follows an unusually deep branch, the method can test whether it is better explained by interbreeding with a population that diverged long before the ancestors of the people being studied.
The important feature is that TRACE does not require a genome from the archaic group itself. In validation tests it recovered major patterns of Neanderthal and Denisovan introgression that are already known from direct ancient-DNA comparisons. Science Official described the study as a way to investigate populations that may have left genetic descendants but no recoverable reference genome.
The researchers then identified additional segments consistent with ancestry from unidentified, or “ghost,” hominin groups in African and non-African populations. In Oceanian genomes, which already contain substantial Denisovan ancestry, TRACE also found very deep signals consistent with a still older contribution that may have entered modern humans indirectly through Denisovan populations.
The word “ghost” is useful but easy to misunderstand. It does not name a newly discovered species. A statistical signal cannot tell us what an extinct population looked like, what language it used or what archaeological culture it belonged to. It indicates that parts of a genome fit a model in which an unknown, deeply diverged population contributed DNA at some point in the past.
That distinction matters for the larger human story. Fossils, artefacts and genomes answer different questions. A skull can reveal anatomy; a stone tool can show behaviour; a genetic segment can preserve a biological relationship even when every bone from the contributing population has disappeared. No single kind of evidence is complete on its own.
TRACE therefore makes the human past more complex rather than more certain. The old image of evolution as a clean tree with separate branches has already given way to a network in which populations met and exchanged genes. Methods that can detect ancestry without reference genomes suggest that some of those connections are still hidden. Future ancient-DNA discoveries may eventually give names or bodies to some of these ghost signals. Others may remain visible only as fragments carried forward in living genomes.