The middle part of the Pleistocene Era spans the time between late Homo erectus (the first hominid species to reach brain sizes overlapping the lower end of human variation) and the appearance of early modern humans in Africa and Neanderthals in Western Eurasia. So few fossils represent the intervening 630,000 years that anthropologists have come to call this period the “muddle in the middle” with much controversy still surrounding their species attributions.
But some inroads are being made – particularly via the most recent paper by Anthony Pagano, PhD., an assistant professor at the Hackensack Meridian School of Medicine.
The nasopharynx or postnasal airway exhibited evolutionary changes among hominids over the last million years, leaving Homo sapiens and Homo neanderthalensis (our cousins, the Neanderthals) with shorter, broader passages, according to the paper in a special issue of The Anatomical Record, a Wiley journal.
This space is of great functional importance and evolutionary change in its anatomy impacts breathing, speech and ventilation of the middle ear cavity. The presence of some modern human and Neanderthal traits among older specimens may also reveal clues on the origins of these groups, according to the paper.
“This is the first study to utilize the nasopharynx as a source of traits in creating an evolutionary history of modern humans and Neanderthals through the murky mid-Pleistocene fossil record,” said Pagano.
The scientists looked at fossils from the Pleistocene and compared them to modern humans. The species from prehistory included H. sapiens and H. neanderthalensis being the youngest, H. erectus representing the oldest, and other fossils of intermediate age whose species attribution remains controversial.
Shape differences among the fossils were assessed using coordinate-based morphometric methods. These involved both univariate measures such as distances and angles and multivariate statistical analysis of the coordinate data. Fossil specimens were assessed alongside a modern human growth series of crania ranging from neonates to adults in age.
The findings became noteworthy because they confirm humans and Neanderthals are different species – while also showing that the latter evolved because of a long period of isolation. The presence of some Neanderthal traits in the nasopharynges of half-million-year-old specimens from Western Europe supports this hypothesis.
The paper fits within other recent Pagano publications, focused on these passages of the ear, nose, and throat.
A 2019 paper in the same journal hypothesized that more horizontal Eustachian tubes (which connect the middle ear to the postnasal airway) allowed for more ear infections among Neanderthal children who do not experience a vertical reorientation of the tube in childhood that is associated with reduced incidence in modern humans. This may have compromised evolutionary fitness among the Neanderthals relative to invasive modern human populations, and ultimately contributed to their extinction. Another publication last year connected a developmental mismatch in the timing of growth of the Eustachian tube and dilator tubae muscle (the muscle that opens it during swallowing) to the timing of peak middle ear disease incidence around one year of age among contemporary human infants.
Pagano said the cumulative work points the way toward understanding where we’ve come from – and exactly what’s changing within our skulls.
“My hypothesis is that the common ancestor of modern humans and Neanderthals deviated from the ancestral condition of Homo erectus (characterized by primitively tall-narrow upper airway shape) by expressing shorter, broader postnasal airways,” he said. “Humans and Neanderthals began diverging in the Middle Pleistocene as evidenced from nasopharyngeal morphology.”