The shoulders of 'Homo antecessor' and modern humans are similar

The shoulders of 'Homo antecessor' and modern humans are similar

The CENIEH has published a paper in the journal Scientific Reports which concludes that Homo antecessor had a shoulder development analogous to that in H. sapiens, although its growth was faster
Homo antecessor shoulders
Homo antecessor scapulae. Credits: D. Garcia Martínez et al

The shape of our shoulders was already present in the Lower Pleistocene, according to a pioneering study published today in the journal Scientific Reports, carried out by Daniel García Martínez and José María Bermúdez de Castro, paleoanthropologists at the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH), in collaboration with David Green of Campbell University (USA).

Studying the shoulder (technically known as the "shoulder girdle") furnishes information on points significant for human evolution such as locomotion, body shape, the possibility of climbing with ease or the ability to launch objects like stones or spears with high accuracy.

The authors of this work were able to study for the first time shoulder growth and development in the species Homo antecessor, dated to 850,000 years old, using tools from virtual anthropology and 3D geometric morphometry. The results show that the course of development of the shoulder in this species was very similar to that in H. sapiens, although the growth might have been faster.

Almost one million years ago, our evolution had already attained almost all the biomechanical capacities characterizing the shoulder in modern humans, and it had definitively parted ways from the abilities still then retained by the more archaic species of the human phylogeny, including climbing with great agility.

 To verify the changes undergone by this part of our anatomy, we need a flat bone: the shoulder blade or scapula. But, as the authors of this study state, “The fossil record of our phylogeny contains barely a handful of these highly delicate bones, which has posed enormous difficulties to studying the growth and development of the shoulders during human evolution”.

Two key fossils

By good luck, at level TD6 of the Gran Dolina site, situated in the Sierra de Atapuerca (Burgos), two scapulae have been conserved: one from a child and the other from an individual of age equivalent to a modern adolescent. These fossils were recovered during the excavation in the first decade of the twenty-first century and belonged to the species H. antecessor.

“In an earlier study of these two fossils, it had been noticed that the morphology of the scapulae was similar to our own. But until now, the growth and development model for the shoulders had remained unknown, and this work has now allowed us to check that our shoulder girdle bones have undergone modifications in accordance with different capacities”, says Bermúdez de Castro.

Comparative study

With the scant information available, it was known that the scapulae of Australopithecus species were similar in some ways to those of chimpanzees or gorillas but were different from our own. “We know that the development of our most archaic ancestors was very similar to that of the anthropoid apes, and the morphology of their shoulders shows that they still retained the capacity to climb with ease. We, on the contrary, have lost this ability”, explains García Martínez.

Comparative of scapulae. Credits: D.García Martínez et al

To determine when our anatomical peculiarities arose, in addition to virtual anthropology and 3D geometric morphometry, the researchers used complex statistical methods to study the development of the shoulder girdle in the species H. antecessor, comparing it with other species from the Pliocene and Lower Pleistocene, such as Australopithecus sediba and A. afarensis. A very broad sample from H. sapiens and chimpanzees (Pan troglodytes) was also used.

“This study shows that although there exist slight morphological differences between the scapulae of H. antecessor and H. sapiens, the former were much more similar to modern humans, to H. erectus and even to Australopithecus than to chimpanzees”, comments García Martínez.

With regard to how the scapulae grew, it was also seen that this was very different from what happens in chimpanzees, and comparable with H. sapiens. “However, it is true that the data seem to point to growth being more rapid in H. antecessor, as highlighted by the CENIEH research team on the basis of dental evidence”, emphasizes Bermúdez de Castro.

This paper lays the foundations for how the shoulder girdle developed in Lower Pleistocene species, and opens the door to new research studying shoulder development in fossil species, as it may become possible to expand the timeframe and study this development even in Pliocene species like the genus Australopithecus.

Full bibliographic information

García-Martínez, D., Green, D., Bermúdez de Castro, J.M. 2021. Evolutionary development of the Homo antecessor scapulae (Gran Dolina site, Atapuerca) suggests a modern-like development for Lower Pleistocene Homo. Scientific Reports. DOI: 10.1038/s41598-021-83039

 

Press release from CENIEH


molar size hominins

New study of molar size regulation in hominins

New study of molar size regulation in hominins

The Dental Anthropology Group at the CENIEH has tested the inhibitory cascade model to see whether it explains the size relationships and differences in shape between the different kinds of teeth, in the molar sample from the individuals identified at the Sima de los Huesos site in the Sierra de Atapuerca.
Mandíbula AT-1 de la Sima de los Huesos. Credits: Mario Modesto

The molar size relationship is one of the peculiar characteristics of the different species of hominins and various theories have been proposed to account for this, as well as the differences in shape between the different kinds of teeth (incisors, canines, premolars and molars). The latest theory, called the inhibitory cascade model, arose out of experiments with mice embryos, and in 2016 it was applied theoretically to fossil hominins, with satisfactory results.

It appeared that all hominins satisfy the inhibitory cascade model. In a paper by the Dental Anthropology Group at the Centro Nacional de Investigación sobre la evolución Humana (CENIEH), published recently in the Journal of Anatomy, this model was tested on the molar sample from the individuals identified at the Sima de los Huesos site, situated in the Sierra de Atapuerca (Burgos).

The results match the model generated in mice extraordinarily well, thus confirming the theory's utility once more. “Nevertheless, our conclusions have brought out an anomaly in the model, when it is applied to the oldest species of the genus Homo”, explains José María Bermúdez de Castro, Paleobiology Program Coordinator at the CENIEH and lead author of this work.

Increasing and decreasing patterns

In the genera Ardipithecus, Australopithecus and Paranthropus, as well as in Homo habilis, the size pattern is increasing and fits the premises of the inhibitory cascade model perfectly. The same thing happens in Homo sapiens, except that the pattern is decreasing, with the first molar larger than the second, which in turn is bigger than the third one (wisdom tooth).

Application of the inhibitory cascade model had led to the assumption that the switch from increasing to decreasing pattern would have arisen a little under two million years ago, perhaps coinciding with the transition between the genera Australopithecus and Homo. “However, in our work we noticed that this change could have required at least a million years to take place”, states Bermúdez de Castro.

The hominins from the Sima de los Huesos, which are around 430,000 years old, are a good example of that transition, whereas most specimens of Homo ergaster, Homo erectus, Homo antecessor and Homo heidelbergensis, as well as other species, do not fit the inhibitory cascade model. “Our idea is to continue our research to determine which genetic mechanisms lie behind this anomaly in the model”, says Bermúdez de Castro.

Full bibliographic information

Bermúdez de Castro et al. 2020. Testing the inhibitory cascade model in the Middle Pleistocene Sima de los Huesos (Sierra de Atapuerca, Spain) hominin sample. Journal of Anatomy. DOI: 10.1111/joa.13292
Press release from CENIEH.

molars Sima de los Huesos

The molars from Sima de los Huesos site share dental tissue traits with Homo antecessor and Neanderthals

The molars from Sima de los Huesos site share dental tissue traits with Homo antecessor and Neanderthals

The Dental Anthropology Group from CENIEH publishes a paper in PLOS ONE in which microscopy and micro-computed tomography are used to study the dental tissues in molars from European Middle Pleistocene individuals found at this site in Atapuerca, and compares these with species from the fossil record and modern humans
Distribution of enamel thickness in a lower molar from Sima de los Huesos compared with H. antecessor, Tighenif specimen and modern human. Credits: Martín-Francés et al.

The Dental Anthropology Group of the Centro Nacional de Investigación sobre la Evolución Humana (CENIEH) has published a paper this week in the journal PLOS ONE which marks another step forward in characterizing the individuals from the Sima de los Huesos site (Atapuerca, Burgos, Spain) and their relationship with Neanderthals and Homo antecessor, and helps to clarify the evolutionary steps that led to the dentition characteristic of Late Pleistocene hominins.

In this paper, whose lead author is the researcher Laura Martín-Francés (CENIEH and PACEA-University of Bordeaux), the dental tissues in the molars of the European Middle Pleistocene individuals found at Sima de los Huesos are analyzed, and compared with species in the fossil record and modern humans.

To conduct this comparative study, micro-computed tomography (mCT) and high-resolution images were used to examine the internal structure of 72 upper and lower molars from this site at Atapuerca, and these were contrasted against another 500 molars belonging to species from the genus Homo, extinct and extant, from Africa, Asia and Europe.

In the entire fossil record analyzed, only the Neanderthals present a unique structural pattern in molar tissues (enamel thickness, percentage of tissues and their distribution in the crown) which, in addition, they do not share with any other species. “In comparison with that record and with modern humans, Neanderthals had thin enamel, with a higher proportion of dentine and a more disperse distribution pattern”, says Martín-Francés.

It has been possible to determine that the molars from the Sima de los Huesos individuals had thick enamel and that, therefore, they do not share this trait with Neanderthals. Nevertheless, the two groups do share the same tissue distribution pattern.

“The results suggest that even though the complex of typically Neanderthal traits appeared later, certain aspects of the Neanderthal molar structure were already present in the hominins from Sima de los Huesos. In earlier work, we had identified this same pattern in Homo antecessor, another of the species recovered at Atapuerca”, adds Martín-Francés.

The Sima de los Huesos population, related genetically to the Neanderthals, represents a unique opportunity to study the appearance of the “typical” structural pattern of Neanderthal molar tissue.

Distribution of enamel thickness in an upper molar from Sima de los Huesos compared with H. antecessor, Neanderthal and modern human. Credits: Martín-Francés et al.

Full bibliographic information

Martín-Francés, L., Martinón-Torres, M., Martínez de Pinillos, M., García-Campos, C., Zanolli, C., Bayle, P., Modesto-Mata, M., Arsuaga, J. L., & Bermúdez de Castro, J. M. (2020). Crown tissue proportions and enamel thickness distribution in the Middle Pleistocene hominin molars from Sima de los Huesos (SH) population (Atapuerca, Spain). PLoS ONE, 15(6), e0233281. doi: 10.1371/journal.pone.0233281.
Press release from CENIEH