Эрдэмтэд хүний геном дахь Неандертал болон Денисовын төрөл зүйлээс гадна өөр олон эртний бүлгийн ДНХ-ийн ул мөрийг илрүүлснээ зарлав.
UC Berkeley-ийн судлаачид Аржун Бидданда болон Юлин Жан нар орчин үеийн хүний геном дахь Неандертал, Денисовын хүнээс өөр, тодорхойлогдоогүй эртний бүлгүүдийн ДНХ-ийн сегментүүдийг хайх судалгааг хийжээ. Тэд “TRACE” хэмээх шинэ аргачлалыг боловсруулж, хүний геном дахь эртний удамшлын хувь нэмрийг нарийвчлан судалсан байна. Судалгааны үр дүнд эдгээр “сүнс” буюу үл мэдэгдэх удамшлын хэд хэдэн үйл явц хүний хувьслын түүхэнд шингэсэн болохыг тогтоожээ.
Судалгаагаар Африкаас гарах нүүдлийн өмнө бий болсон, өнөөгийн бүх хүн төрөлхтөнд нөлөөлсөн удамшлын тодорхой ул мөрийг илрүүлсэн байна. Энэхүү эртний бүлэг нь ойролцоогоор нэг сая жилийн өмнө амьдарч байсан байж болзошгүй бөгөөд тэдний ген 300,000 жилийн өмнө эрлийзжих замаар орчин үеийн хүний өвөг дээдэст нэвтэрчээ. Мөн Океанийн бүс нутгийн хүн амын Денисовын удамшлаас бүр ч эртний бүлгийн ДНХ-ийн ул мөр олдсон нь хүн төрөлхтний хувьслын түүх урьд өмнө төсөөлж байснаас илүү ээдрээтэй болохыг харуулж байна.
Гэсэн хэдий ч энэхүү эртний ДНХ-ийн эх үүсвэр яг ямар төрөл зүйл болохыг одоогоор эцэслэн тогтоогоогүй байна. Судлаачид Homo erectus болон Homo heidelbergensis-ийг нэр дэвшигчээр дэвшүүлж байгаа ч олдворын бүртгэлд байхгүй эсвэл геномд илрэх боломжгүй бусад эртний бүлгүүд байх магадлалтай гэж үзэж байна. Эрдэмтэд эдгээр эртний генийн дархлаа болон бодисын солилцоонд үзүүлэх нөлөөг цаашид судлах шаардлагатайг онцолжээ.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
Scientists already know that Homo sapiens interbred with Neanderthals and Denisovans, leaving recognizable traces in people alive today, but those groups do not explain every archaic segment of the human genome. Some sequences point to populations for which researchers have no matching genetic identity, the so-called ghost lineages.
At UC Berkeley, postdoctoral researcher Arjun Biddanda and graduate student Yulin Zhang set out to find these harder-to-identify regions. Their analysis, published in Science, searched for ancient ancestry that could not be attributed to Neanderthals or Denisovans. The results point not to a single unexplained encounter, but to several ancient contributions embedded in the genetic history of modern humans.
Ghost Ancestry Reached Everyone
One of the clearest signals appears to come from an unidentified population that contributed DNA to human ancestry before the out-of-Africa expansion, leaving a genetic legacy that would later spread across populations as modern humans migrated into different regions of the world.
“We find evidence for at least one ghost introgression predating the [out of Africa] expansion that contributed ancestry to all present-day populations,” wrote the authors in Science.
Previous work had left open whether this particular ghost ancestry was confined to African populations. The new analysis extends that picture beyond West Africans, indicating that the genetic contribution affected all modern human populations. As Popular Mechanics reports, the lineage from which modern humans evolved is thought to date back roughly 550,000 to 750,000 years, while the ghost species associated with the mysterious ancestry probably lived around one million years ago.
One ghost lineage identified in the analysis appears to have diverged around the period when modern humans split from Neanderthals. Its genetic material likely entered human ancestry about 300,000 years ago through introgression, the transfer of genes between species through interbreeding.
TRACE Reveals More Than One Ancient Genetic Encounter
To uncover these signals, UC Berkeley researchers developed TRACE, short for TRacking Archaic Contributions via ARG Estimation. The framework was designed to identify super-archaic regions in modern genomes that cannot be explained by known Neanderthal or Denisovan ancestry. The results revealed evidence consistent with multiple introgression events involving unidentified archaic groups, alongside the expected genetic contributions from Neanderthals and Denisovans.
The analysis also uncovered traces of even older lineages within the Denisovan ancestry of Oceanian populations. These findings strengthen the evidence connecting Homo sapiens to a super-archaic hominin species, indicating that some genetic material preserved in modern humans has a deeper and more complex history than known Neanderthal and Denisovan contributions.
How this ancient DNA ultimately reached modern humans remains unresolved. The researchers describe two possible routes: unidentified hominins may have interbred directly with early Homo sapiens or their ancestors, or unknown archaic populations may first have interbred with Neanderthals or Denisovans, whose descendants later passed those older sequences to modern humans. Some of the oldest DNA found in people today may therefore have traveled through another hominin lineage before entering the modern human genome.
Two Ancient Human Species Emerge as Candidates
The genetic signatures reveal unidentified ancestry, but they do not establish exactly which ancient hominins produced it. Biddanda and Zhang identify Homo erectus and Homo heidelbergensis as possible candidates. For now, the available genetic evidence cannot securely connect the ghost ancestry to either species. Other hominins may also be involved. Some populations responsible for these signals could be missing from the fossil record, while others may simply remain undetectable in our genes.
Researchers have yet to determine precisely where and when all the interbreeding events occurred. Archaic hominin ancestry accounts for about 2% of the Homo sapiens genome, as reported by Popular Mechanics, but the identities behind some of those ancient contributions remain unresolved. Their biological effects present another open question.
“[Ghost genes’] roles inimmunityand metabolism have yet to be fully characterized,” Biddanda and Zhang noted. “Future studies that integrate additional ancient genomes at deep timescales, particularly from Africa and Asia, will be critical for refining the timing, sources, and evolutionary consequences of these introgression events.”
Researchers believe much older genomes from Africa and Asia could narrow down when these introgression events took place and reveal which ancient populations were involved. They could also show how these genetic exchanges influenced human evolution and help uncover the origins of ghost DNA that survives in people today.
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