Ghost DNA and Human Evolution

Coauthored with the New Bing

What is a ghost lineage in terms of human evolution? How is ancient DNA obtained, and how do we know who is missing? What evidence do we have to support whom we think the species is? Has the species been identified? How long ago did this introgression of DNA occur? What does it mean for our genetics today?

A ghost lineage is a hypothesized ancestor in a species lineage that has yet to leave any fossil evidence yet can be inferred to exist because of gaps in the fossil record or genomic evidence. [1] The process of determining a ghost lineage relies on fossilized evidence before and after the hypothetical existence of the lineage and extrapolating relationships between organisms based on phylogenetic analysis. [2] Ghost lineages assume unseen diversity in the fossil record and serve as predictions for what the fossil record could eventually yield; these hypotheses can be tested by unearthing new fossils or running phylogenetic analyses. [3]

Ancient DNA is obtained from fossils or other preserved remains of extinct organisms, such as bones, teeth, hair, skin, or shells. [4] Ancient DNA can reveal information about the genetic relationships, evolutionary history, and demographic patterns of extinct lineages. [4] However, ancient DNA is often degraded, contaminated, or scarce, making it challenging to analyze and interpret. [4]

We do not know who is missing from the fossil record because fossils are rare and incomplete. Fossils only form under certain conditions, such as rapid burial and mineralization, and they are often destroyed by erosion, weathering, or human activity. [5] Therefore, the fossil record is biased towards certain organisms, environments, and periods and does not reflect the true diversity and complexity of life on Earth. [5]

One way to infer the existence of ghost lineages is to compare the genomes of modern humans with those of extinct human relatives, such as Neanderthals and Denisovans. [6] By identifying segments of DNA that are shared or different among these groups, we can estimate when they diverged from each other and when they interbred. [6] For example, researchers found a “ghost population” that interbred with the ancestors of modern West Africans by detecting statistical anomalies in their genomes that were best explained by hybridization with an unknown ancient human lineage. [6]

The ghost population that interbred with West Africans has yet to be identified, as no fossils or ancient DNA samples have been found that belong to this lineage. [6] However, based on genomic analysis, the researchers estimated this ghost lineage diverged from the ancestors of Neanderthals and modern humans up to 1.02 million years ago and interbred with the ancestors of modern West Africans from 124,000 years ago up to the present day. [6]

The introgression of DNA from ghost lineages into modern humans may have had various effects on our genetics today. For example, some genes from Neanderthals and Denisovans may have conferred adaptive advantages to modern humans in certain environments or against certain pathogens. [7] On the other hand, some genes from extinct lineages may have been incompatible with modern human genes or reduced fertility in hybrids. [7] Moreover, some ghost lineage genes may not significantly affect current human traits or functions. [7]

So what does this all mean? It is not just possible, but more than likely, that other hominins have yet to be discovered, which have played an essential role in our evolution. We can learn more about this possible enigmatic species when we get aDNA. Being able to find DNA, which we can now even gather from sediments, we can learn a great deal about where we came from. But once we have that DNA, we can determine who these ghost lineages are, and with that information, the mystery will continue.

There are many gaps in the fossil record; the only way to fill those gaps is by going into the field and exploring and discovering what there is to learn. DNA can reveal so many mysteries about our ancestors and help show what is in store for us in the future.

References:

[1] Ghost lineage – Wikipedia

[2] University of California Museum of Paleontology – Ghost lineages

[3] Nature – Ghosts in the machine

[4] Nature Education – Ancient DNA: The first three decades

[5] Understanding Evolution – The nature of the fossil record

[6] Inside Science – Genetic Traces of Mysterious Human Lineage Detected in People Living in West Africa

[7] Nature – The genomic landscape of Neanderthal ancestry in present-day humans