Homo Sapiens: Anatomically Modern Humans (AMH)

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Homo Sapiens: Anatomically Modern Humans (AMH) Apr 10, 2021 20:31:39 GMT

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Post by Admin on Apr 10, 2021 20:31:39 GMT

The Dmanisi endocasts
The endocast of the adult individual D2280 (Fig. 2A, fig. S1A, and supplementary text S1) is generally well preserved but lacks parts of the basicranial region. It has a rounded overall shape, with a comparatively wide anterior cranial fossa, and right occipital and left frontal petalia. The cerebellar fossa is large and bulges inferiorly. Imprints of the frontal sulci course toward the precentral sulcus (pc). This latter structure originates near the apex of the endocast, crosses the coronal suture at mid-height, and courses toward BC, such that its inferior portion (pci) lies anterior to the coronal suture.

Fig. 2 Endocranial organization of the Dmanisi crania.
(A to E) Left and right lateral, posterior, and superior views of (A) D2280, (B) D2282, (C) D2700, (D) D3444, and (E) D4500. Red, sulcal imprints; blue, sutures. Abbreviations are as in Fig. 1. SA, sagittal suture. Scale bar, 5 cm. (See also fig. S2.) In all specimens, the precentral sulcus (pc) crosses the coronal suture (CO), such that its inferior portion (pci) lies anterior to CO, indicating a primitive organization of the frontal lobe (see Fig. 1A).

The cranium of the adult individual D2282 (Fig. 2B, fig. S1B, and supplementary text S1) is fragmentary and exhibits substantial taphonomic distortion, which required digital retrodeformation and reconstruction. Petalial asymmetry cannot be ascertained. Several key endocranial structures are nevertheless visible. Similar to D2280, the endocranium is rounded and exhibits wide frontal lobes. The cerebellar fossa is only moderately bulging. On the left side, the precentral sulcus crosses the coronal suture at mid-height and courses toward the center of BC.

The endocranial cavity of the adolescent individual D2700 (Fig. 2C, fig. S1C, and supplementary text S1) is generally well preserved. Structural detail in the parietal and occipital region is blurred by a thin but dense calcite layer, which tightly adheres to the bone surface. In lateral and posterior views, the endocast appears rounded; in superior view, it presents a marked precoronal constriction, a feature that is often seen in Asian fossils attributed to H. erectus. The endocast exhibits left frontal petalia. The occipital poles are moderately projecting, and the cerebellar fossa is only moderately bulging. On both sides, the imprint of the medial frontal sulcus courses toward the coronal suture, where it reaches the precentral sulcus. The inferior portion of the precentral sulcus (pci) lies in front of the coronal suture and courses toward BC. Preservation of the endocranial base region permits estimation of cranial base angulation. The angle between landmarks basion, sella, and foramen caecum is 135° [cranial base angle CBA1 (36)]. The angle between the clivus plane and the midplane of the anterior cranial fossa (planum sphenoideum) is 124° [cranial base angle CBA4 (36)]. These values are at the upper end or above the range of variation of modern humans of similar dental age [CBA1, 132° to 137°; CBA4, 106° to 118° (37)].

The endocranial cavity of the edentulous cranium D3444 (Fig. 2D, fig. S1D, and supplementary text S1) suffered some taphonomic damage, such as dislocation of the left temporal pyramid, and loss of parts of the anterior cranial base and of the internal table on the left fronto-parietal region. The endocast has a globular shape that is similar to that of some modern human endocasts (Fig. 3) but exhibits precoronal constriction similar to D2700. The endocast exhibits slight right frontal petalia. The frontal sulci are represented by marked endocranial imprints, which course toward the imprint of the precentral sulcus. On both sides, the pci lies anterior to the coronal suture and courses toward the center of BC.

Fig. 3 Endocranial shape variation in fossil hominins, great apes, and humans.
The central graph shows patterns of intragroup and intergroup variation in shape space. The surrounding panels visualize endocranial shapes at the extremes of the distribution. Shape component SC1 accounts for 57% of the total variation in the sample and captures major differences in endocranial shape between great apes and hominins, notably in relative width of frontal lobes (arrows) and in foramen magnum position. Shape component SC2 (10%) captures variation in endocranial length, height, and width relationships (arrows) within and among groups. Green stars, Pongo pygmaeus; red, Pan troglodytes (open squares, P. t. troglodytes; horizontal rectangles, P. t. verus; vertical rectangles, P. t. schweinfurthii); orange dots, P. paniscus; blue diamonds, Gorilla gorilla; filled circles, Australopithecus; open circles, African early Homo; triangles, Asian early Homo (left, upward, and downward triangles indicate Dmanisi, Northeast Asia, and Southeast Asia, respectively); + signs, mid- to late-Pleistocene African and European Homo; × signs, Homo neanderthalensis; black squares, modern Homo sapiens. Light and dark hues indicate immature and adult individuals, respectively. The 90% density ellipses are drawn for great ape species, modern H. sapiens, early Homo between 2 and 1 Ma (black outline), and H. neanderthalensis. For fossil specimen labels, see Table 1. Hexagons indicate the location in shape space of the visualized endocasts.

Cranium D4500, together with mandible D2600, represents the most complete skull of early Homo found to date (32). The endocast is fully preserved, except for a small area of the internal table in the occipital region (Fig. 2E, fig. S1E, and supplementary text S1). Sediment adhering to the middle and anterior cranial fossae was removed using synchrotron imaging and semiautomated image segmentation procedures (supplementary method M2). The D4500 endocast exhibits right occipital petalia. Precoronal constriction is marked, but the frontal lobes remain relatively wide. The frontal sulci are represented by marked imprints; they course toward the precentral sulcus, which is clearly represented on both sides of the endocast. The precentral sulcus crosses the coronal suture at mid-height, such that pci is located anterior to the coronal suture and courses toward BC. The occipital poles are protruding. The internal cranial base of D4500 is substantially less flexed than that of D2700, with a CBA1 of 156° and CBA4 of 144°. Both values are outside the range of variation of adult modern humans.

Altogether, the Dmanisi endocasts indicate a consistent topographic pattern of external cortical morphology, where the precentral sulcus crosses the coronal suture such that its inferior portion is anterior to the suture and courses toward BC (Fig. 2, Table 1, and fig. S1). Using great ape and human cranio-cerebral topographies as a reference (Fig. 1), the Dmanisi individuals largely reflect a great ape pattern of frontal lobe organization, in which BC is delimited inferiorly by the fronto-orbital sulcus and likely housed Brodmann cortical area 44 and parts of area 45 (Fig. 1A).

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Homo Sapiens: Anatomically Modern Humans (AMH) Apr 11, 2021 0:42:40 GMT

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Post by Admin on Apr 11, 2021 0:42:40 GMT

Endocasts of African and East Asian Homo fossils
A comparative analysis of the endocasts of early to mid-Pleistocene Homo fossils from Africa reveals greater endocranial topographic diversity (Table 1, supplementary text S2, and fig. S2). Specimens KNM-ER1805 and KNM-ER1813, with disputed taxonomic affiliation but dated to before ~1.7 Ma, show endocranial imprints that are compatible with a great ape–like organization of the frontal lobe (fig. S2, A and B). Specimen KNM-ER 1470 (fig. S2C), which has been put forward as key evidence for a derived frontal lobe organization (21), is too fragmented to reveal diagnostic endocranial imprints. The endocasts of fossils dated to ~1.7 to 1.5 Ma, and variably attributed to Homo ergaster or H. erectus, show variation in frontal endocranial topography that suggests a range of primitive to derived frontal lobe morphologies (fig. S2, D to F). By contrast, African fossils younger than ~1.5 Ma exhibit endocranial topographies that are compatible with a derived, modern human–like frontal lobe organization (Table 1 and supplementary text S2).

The endocasts of H. erectus from Java typically exhibit marked imprints of the frontal sulci, whereas imprints of the precentral and central sulci are faint to absent. In all specimens reexamined here, imprints of the inferior and/or middle frontal sulci course anteroposteriorly to reach the coronal suture (Table 1, supplementary text S3, and fig. S3, A to E). This topography constrains the inferior precentral sulcus to a location that either roughly coincides with the coronal suture or is posterior to it, thus representing a derived organization of the frontal lobe. The endocasts of Chinese H. erectus, represented here by Zhoukoudian XII (fig. S3F) and Hexian, exhibit a clearly derived topography of the frontal lobe region (supplementary text S3).

Endocranial shape, size, and topography
Figure 3 shows endocranial shape variation in fossil Homo and in a comparative sample of great apes and modern humans (see supplementary method M4). Great ape endocrania are low but wide in the parietal region and exhibit a tapering frontal region and a posteriorly situated foramen magnum. Modern human endocrania are comparatively high and have wide frontal lobes and an inferiorly situated foramen magnum. The great ape–human contrast in endocranial shape is largely an effect of variation in relative encephalization [measured here as brain size relative to the size of the cranial base and face (38)] (fig. S4). Fossil hominin endocrania occupy the region in morphospace between great apes and humans. Endocranial shape in the genus Homo between 2 and 1 Ma shows wide intragroup and intergroup variation (Fig. 3). Variation in the Dmanisi sample is congruent in range and pattern with intragroup variation in other taxa (fig. S5).

Correlating our data about endocranial topography (Table 1) with geological dates permits inferences on the timing of frontal lobe reorganization and its possible correlation with brain expansion and changes in brain shape (Fig. 4). The transition from primitive to derived frontal lobe morphologies likely took place during the time between 1.7 and 1.5 Ma, with derived morphologies largely established in the fossil record after 1.5 Ma. During that time interval, frontal lobe reorganization occurred in tandem with brain expansion (Fig. 4A and Table 1) [mean endocranial volume (ECV) increased from 650 to 830 cm3]. It is likely that the two processes had similar evolutionary causes. However, the possible presence of derived frontal lobe morphologies in small-brained (ECV ~460 cm3) but geologically younger fossils, such as H. floresiensis and H. naledi (9) (supplementary text S4), suggests that frontal lobe organization is not mechanistically linked to large brain volumes.

Fig. 4 Evolution of endocranial morphology of early Homo.
(A) Changes in ECV and craniocerebral topography over the past 2 Ma. Orange, gray, and turquoise symbols indicate the position of the inferior precentral sulcus (pci) anterior to, coincident with, and posterior to the coronal suture (CO), respectively (see inset graph). White symbols indicate missing or indeterminate data. Circles indicate Africa, left-facing triangles indicate Western Asia and Europe, and upward and downward triangles indicate Northeast and Southeast Asia, respectively (see inset world map). Dashed symbols indicate immature specimens, and horizontal lines indicate geological age ranges. (B) Endocranial shape change associated with frontal lobe reorganization (left lateral view). Colors denote above-average expansion (green) and bulging (yellow) of the endocranial surface, indicating differential enlargement of labeled brain regions (IPF, inferior prefrontal cortex; 45/47, Brodmann areas 45/47; PP, posterior parietal cortex; T, temporal lobe, O, occipital lobe). Also note endocranial surface expansion along the transverse sinus (TS). Brain sulci (red): fs/fi, superior/inferior frontal; pc/c/pt, precentral/central/postcentral; s, Sylvian; ip, intraparietal.

Our data further indicate that the transition from primitive to derived frontal lobe organization was accompanied by specific changes in endocranial shape (Fig. 4B and supplementary method M5). The posterior shift of pci (Fig. 4A) is associated with differential expansion of the inferior prefrontal (IPF) region (Figs. 1 and 4B). Furthermore, expansion of IPF is correlated with marked expansion of the posterior parietal (PP) and occipital (O) cortical regions.

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Homo Sapiens: Anatomically Modern Humans (AMH) Apr 11, 2021 2:44:59 GMT

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Discussion
Pattern and timing of brain reorganization in early Homo
Cranio-cerebral topography reveals that the earliest members of the genus Homo had a primitive frontal lobe organization, featuring an ape-like anterior location of the inferior precentral sulcus relative to the coronal suture (Fig. 4A and Table 1). Our data indicate that the derived frontal lobe organization emerged relatively late during the evolution of Homo, between 1.7 and 1.5 Ma—not at the transition from Australopithecus to Homo (10, 39), but clearly later than the first dispersals of Homo from Africa. Endocranial shape change associated with frontal lobe reorganization reveals differential expansion of the inferior prefrontal cortex and also of the posterior parietal and occipital cortex (Fig. 4B) (39). This pattern indicates that the anterior and posterior cortical association areas evolved in tandem (13, 40, 41) rather than in sequence (3). We infer from this that endocasts of early Homo predating frontal lobe reorganization potentially exhibit imprints of remnant ape-like lunate sulci in the parieto-occipital region (supplementary text S1 and fig. S1).

The temporal and geographic patterning of primitive and derived brain organization in early Homo (Table 1) cannot be explained by a single-dispersal scenario but must have involved greater spatiotemporal complexity, as suggested earlier (42–45). Given the current evidence (Fig. 4 and Table 1), the most parsimonious scenario is that the first Homo populations to disperse from Africa, probably as early as 2.1 Ma (46), retained the primitive frontal lobe organization, as represented in Dmanisi. The Southeast Asian H. erectus fossils, now dated to <1.5 Ma (26), represent a second dispersal, after the derived frontal lobe morphology emerged in Africa between 1.7 and 1.5 Ma. Additional fossil and archaeological evidence will be required to assess whether the earliest populations of Homo outside Africa merged with, and/or were replaced by, populations exhibiting the derived morphology.

Neurofunctional implications
In modern human brains, the inferior frontal lobe is an important neurofunctional substrate for advanced social cognition, toolmaking and tool use, and articulated language (2, 47, 48). We may thus ask whether its evolutionary reorganization around 1.7 to 1.5 Ma was accompanied by major changes in technocultural performance. The earliest evidence for Mode II (Acheulean) technocultures in Africa [1.76 Ma (49, 50)] largely coincides with incipient frontal lobe reorganization, and Mode I and Mode II lithic technologies were used concurrently during the critical time period (51). We hypothesize that this pattern reflects interdependent processes of brain-culture coevolution (52), where cultural innovation triggered changes in cortical interconnectivity (4, 53) and ultimately in external frontal lobe topography. On the other hand, the cerebral innovations that characterize Homo at ~1.5 Ma might have constituted the foundations of the “language-ready” brain of later Homo species (54).

Taxonomic implications
Our findings also have implications for the taxonomy of early Homo. The notable morphological diversity of early Pleistocene fossils attributed to Homo has been interpreted alternatively as population diversity within a single hominin species lineage versus species diversity in Africa. The former view is supported by patterns of continuous variation in metric and nonmetric cranial traits (32) and by evidence for substantial sexual dimorphism (51, 55). The latter view is supported by the observation that metric variation in gnathic morphology is greater than expected for a single taxon, suggesting dietary specialization rather than brain expansion as a major driver of speciation in early Homo (56). The data on brain structural variation presented here provide additional evidence for diversity in early Homo in Africa. However, the pattern of cerebral structural diversity (Fig. 4A) does not match the pattern of gnathic diversity (56), such that the question of taxonomic diversity in early Homo remains unresolved. Deciphering evolutionary process in early Homo remains a challenge that will be met only through the recovery of expanded fossil samples from well-controlled chronological contexts.

Supplementary Materials
science.sciencemag.org/content/372/6538/165/suppl/DC1

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Homo Sapiens: Anatomically Modern Humans (AMH) May 8, 2021 2:33:57 GMT

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Post by Admin on May 8, 2021 2:33:57 GMT

Humans diverged from apes — specifically, the chimpanzee lineage (genus Pan) — at some point between 9.3 and 6.5 million years ago, and habitual bipedalism evolved early in hominins.

To understand hominin origins, paleoanthropologists aim to reconstruct the physical characteristics, behavior, and environment of the last common ancestor of humans and chimps.

“In The Descent of Man in 1871, Charles Darwin speculated that humans originated in Africa from an ancestor different from any living species. However, he remained cautious given the scarcity of fossils at the time,” said Dr. Sergio Almécija, a researcher in the Division of Anthropology at the American Museum of Natural History.

“150 years later, possible hominins have been found in eastern and central Africa, and some claim even in Europe.”

“In addition, more than 50 fossil ape genera are now documented across Africa and Eurasia.”

“However, many of these fossils show mosaic combinations of features that do not match expectations for ancient representatives of the modern ape and human lineages.”

“As a consequence, there is no scientific consensus on the evolutionary role played by these fossil apes.”

The evolutionary history of apes and humans is largely incomplete: whereas the phylogenetic relationships among living species can be retrieved using genetic data, the position of most extinct species remains contentious; surprisingly, complete-enough fossils that can be attributed to the gorilla and chimpanzee lineages remain to be discovered; assuming different positions of available fossil apes — or ignoring them owing to uncertainty — markedly affects reconstructions of key ancestral nodes, such as that of the chimpanzee-human last common ancestor. Image credit: Almécija et al., doi: 10.1126/science.abb4363.

There are two major approaches to resolving the human origins problem:

(i) ‘top-down,’ which relies on analysis of living apes, especially chimpanzees;

(ii) and ‘bottom-up,’ which puts importance on the larger tree of mostly extinct apes.

For example, some scientists assume that hominins originated from a chimp-like knuckle-walking ancestor.

Others argue that the human lineage originated from an ancestor more closely resembling, in some features, some of the strange Miocene apes.

In reviewing the studies surrounding these diverging approaches, Dr. Almécija and his colleagues discuss the limitations of relying exclusively on one of these opposing approaches to the hominin origins problem.

‘Top-down’ studies sometimes ignore the reality that living apes are just the survivors of a much larger, and now mostly extinct, group.

On the other hand, studies based on the ‘bottom-up’ approach are prone to giving individual fossil apes an important evolutionary role that fits a preexisting narrative.

Overall, the researchers found that most stories of human origins are not compatible with the fossils that they have today.

“Living ape species are specialized species, relicts of a much larger group of now extinct apes,” said Dr. Ashley Hammond, an assistant curator in the Division of Anthropology at the American Museum of Natural History.

“When we consider all evidence, it is clear that a human evolutionary story based on the few ape species currently alive is missing much of the bigger picture.”

“The unique and sometimes unexpected features and combinations of features observed among fossil apes, which often differ from those of living apes, are necessary to untangle which features hominins inherited from our ape ancestors and which are unique to our lineage,” added Dr. Kelsey Pugh, a postdoctoral researcher in the Division of Anthropology and the New York Consortium in Evolutionary Primatology at the American Museum of Natural History.

“Living apes alone offer insufficient evidence. Current disparate theories regarding ape and human evolution would be much more informed if, together with early hominins and living apes, Miocene apes were also included in the equation,” Dr. Almécija said.

“In other words, fossil apes are essential to reconstruct the ‘starting point’ from which humans and chimpanzees evolved.”

A paper on the findings was published in the journal Science: science.sciencemag.org/content/372/6542/eabb4363

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Sergio Almécija et al. 2021. Fossil apes and human evolution. Science 372 (6542): eabb4363; doi: 10.1126/science.abb4363

Last Edit: May 8, 2021 2:34:21 GMT by Admin

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Homo Sapiens: Anatomically Modern Humans (AMH) Jan 12, 2022 21:15:34 GMT

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Post by Admin on Jan 12, 2022 21:15:34 GMT

Scientists say a Homo sapiens fossil found in Ethiopia in the 1960s is at least 233,000 years old, which would make it 36,000 years older than the previous estimate.

Fossils of the earliest members of our species, Homo sapiens, are exceptionally rare. To date, only eight sites in Africa have produced fossils of early anatomically modern humans, the oldest of which, known as the Kibish Omo I fossil, has now been re-dated by team of experts led by Clive Oppenheimer from the Department of Geography at the University of Cambridge.

That’s right—geography, and not archaeology or anthropology. Sure, members of those disciplines were involved in the study, published today in Nature, but the effort to re-date this fossil required scientists of the Earth, specifically experts in volcanology. For you see, the bones of Omo I were found directly beneath a layer of volcanic ash.

Previous efforts to date this ash and to distinguish it from other layers of ash have led to uncertainties about the fossil’s age, but the scientists behind the new study think they’ve finally nailed it. The new minimum age of Omo I is 233,000 years old, as opposed to the previous estimate of 197,000 years old.

“This is the best estimate we have at the moment, and it is congruent with the most recent models of human evolution, which place the emergence of our species—Homo sapiens—between 350,000 and 200,000 years ago,” Céline Vidal, a volcanologist at the University of Cambridge and the first author of the new study, explained in an email. To which she added: “The Omo I fossil is the oldest Homo sapiens we know of so far,” and until now it was “thought to be younger than 200,000 years old, but there was a lot of uncertainty on this age.”

The Omo I skeleton was found in 1967 at the Omo Kibish Formation of southwestern Ethiopia. The site is positioned within the East African Rift valley, an area prone to volcanic activity. Vidal is part of a research project seeking to understand the timing and history of very large eruptions in the Ethiopian Rift between 300,000 and 60,000 years ago, which she described as “poorly studied.” This data, it is hoped, will help scientists to better understand possible links with human evolution.

“We have been analysing volcanic rocks from these eruptions to find out when they occurred and identify their chemical fingerprint,” Vidal wrote. “Each eruption has a unique chemical signature. Using these fingerprints, we could track the far-traveled fine ash hundreds of miles away from the volcanoes. In Ethiopia, these ash layers can be found at archaeological sites where tools and fossils of our ancestors were identified.”

Traditional radiometric, or istopic, techniques are limited when it comes to the dating of ash, but by studying the chemical composition of pumice rock samples from volcanic deposits, the team was able to date all major volcanic eruptions that happened in Africa during the Late Middle Pleistocene—the time when our species first appeared.

“We identified the source of the ash to be a colossal eruption of Shala volcano, which occurred about 233,000 years ago,” Vidal said. “This means that Omo I is older than 230,000 years.”

The margin of error on this new estimate is significant at a plus-minus of 22,000 years, a degree of uncertainty the team hopes to refine in future work. Also, the team wasn’t able to determine the maximum age of the fossil. To that end, Vidal and her colleagues are currently trying to correlate an ash layer located beneath the fossil to other ash deposits.

The new study is interesting, but it’s far from paradigm-shattering. That the oldest known fossil of an anatomically modern human is older than 233,000 years is hardly surprising, given that genetic evidence points to an even earlier emergence of our species, perhaps as far back as 600,000 years ago. For example, certain fossils from the Jebel Irhoud site in Morocco date back some 300,000 years, but they represent archaic, not modern, Homo sapiens.

The Jebel Irhoud fossils “present some characteristics of Homo sapiens but are not considered as complete Homo sapiens,” said Vidal. Omo I, on the other hand, possesses features consistent with anatomically modern humans, such as a tall and round skullcap and a chin on the mandible.

This aside, the new work “is important in demonstrating that the Omo I skeleton is pretty certainly more than 230,000 years old, thus even older than previously suggested,” Chris Stringer, an anthropologist at the Natural History Museum of London who wasn’t involved in the the new study, wrote in an email. “And on the parts preserved, this is an anatomically modern human and thus the oldest known so far. However, it’s likely from genetic data that the Homo sapiens lineage stretches back even further, for at least another 300,000 years, although the earlier members of the lineage would not have shown all the ‘modern’ traits that evolved subsequently.”

Age of the oldest known Homo sapiens from eastern Africa

Published: 12 January 2022

Abstract
Efforts to date the oldest modern human fossils in eastern Africa, from Omo-Kibish1,2,3 and Herto4,5 in Ethiopia, have drawn on a variety of chronometric evidence, including 40Ar/39Ar ages of stratigraphically associated tuffs. The ages that are generally reported for these fossils are around 197 thousand years (kyr) for the Kibish Omo I3,6,7, and around 160–155 kyr for the Herto hominins5,8. However, the stratigraphic relationships and tephra correlations that underpin these estimates have been challenged6,8. Here we report geochemical analyses that link the Kamoya’s Hominid Site (KHS) Tuff9, which conclusively overlies the member of the Omo-Kibish Formation that contains Omo I, with a major explosive eruption of Shala volcano in the Main Ethiopian Rift. By dating the proximal deposits of this eruption, we obtain a new minimum age for the Omo fossils of 233 ± 22 kyr. Contrary to previous arguments6,8, we also show that the KHS Tuff does not correlate with another widespread tephra layer, the Waidedo Vitric Tuff, and therefore cannot anchor a minimum age for the Herto fossils. Shifting the age of the oldest known Homo sapiens fossils in eastern Africa to before around 200 thousand years ago is consistent with independent evidence for greater antiquity of the modern human lineage10.