Researchers from UC Berkeley and Johns Hopkins University have identified genetic traces of two unknown hominin species within the modern human genome .. The study, published in the journal Science, suggests these "ghost ancestors" contributed significantly to our evolutionary history through ancient interbreeding events.
The 800,000-year-old ancestor found in all humans
One of the identified ghost lineages diverged from the human line roughly 800,000 years ago before interbreeding with Homo sapiens approximately 50,000 years ago. According to the study published in Science, this interaction occurred prior to the primary migration out of Africa, which explains why this genetic signature is present in both African and non-African populations today.
Every modern human carries between 0.5% and 1% of this unknown ancestor's DNA, a proportion comparable to the genetic legacy left by Neanderthals. This significant contribution suggests that these ancient interactions were not isolated incidents but were part of a broader pattern of human development.
A 1.8 million-year-old link to Denisovan DNA
A second, much older "super-archaic" hominin dates back nearly 1.8 million years and followed a distinct evolutionary path. Unlike the first ghost ancestor, this species did not interbreed directly with modern humans; instead, it mixed with Denisovans, an ancient group residing in Eurasia.
Because Denisovans later interbred with modern human populations, particularly in Asia, the genetic echoes of this 1.8-million-year-old species persist within Denisovan-derived segments of our current genome. This creates a multi-layered genetic history where one ancient species' influence is filtered through another before reaching modern humans.
How the TRACE method bypasses the need for fossils
The TRACE (TRacking Archaic Contributions via ARG Estimation) method allows scientists to uncover these hidden histories without requiring physical skeletal remains. This novel technique, utilized by researchers at UC Berkeley and Johns Hopkins University, identifies genomic segments that do not align with known species like Neanderthals or Denisovans.
Human evolution appears to be a complex network rather than a simple branching tree , according to the findings. The TRACE method suppots a model where populations repeatedly diverged,migrated, and mixed, creating a "braided" evolutionary history rather than a straight line of descent.
The missing physical evidence for these silent lineages
The absence of physical fossils for these two ghost species remains a significant gap in our understanding of their biological reality. While the TRACE method provides a clear genetic map, the actual physical appearance, stature, and specific habitats of these ancestors remain entirely unknown to science.
Furthermore, while the study suggests that these archaic DNA segments may have provided adaptive advantages in immunity and metabolism, the specific biological benefits are still being investigated. as co-author Yulin Zhang noted, while previous studies hinted at such ancestry, the timing and global distribution of these events were previously impossible to determine with certainty.
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