|
Post by Admin on Mar 24, 2019 18:36:10 GMT
Since the first publication of bulk bone collagen carbon and nitrogen stable isotope ratios of Neandertals, (e.g., refs. 1⇓–3), supported by more recent studies (4⇓⇓⇓–8), an intriguing pattern has been observed: Neandertals are characterized by relatively high nitrogen isotope values, often higher than the carnivores from the same sites (Fig. 1). Nitrogen isotopes are usually employed as a tracer of trophic level (9), and this pattern was therefore, at first, interpreted as indicative of the top-level trophic position (TP) of Neandertals in Paleolithic food webs (2, 3, 10), specifically focused on the consumption of large herbivorous mammals (11). Early modern humans have a wider range of isotopic ratios (especially in carbon), and some individuals exhibit higher N isotope ratios than of the fauna associated with them and above the range generally observed in Neandertals (refs. 10 and 12⇓–14 and Fig. 1). These elevated values in Pleistocene modern humans have been interpreted as the signature of freshwater fish consumption (10, 13) as N isotope ratios (δ15N) in fish or shell consumers are usually higher than in pure terrestrial carnivores (10). Recently, an exceptionally high N isotope ratio has been measured in an infant Neandertal (AR-14) from the site of Grotte du Renne (Arcy-sur-Cure, France; SI Appendix, Supporting Information 1). This individual had a δ15N value 3–5‰ above the associated carnivores (ref. 6 and Fig. 1). In this case, this very elevated ratio is likely to be due to breastfeeding (15), as the infant was ∼1 y old. Fig. 1. δ15N values of Neandertals, Pleistocene modern humans, and fauna from different Paleolithic European and Asian sites for which analyses on the associated fauna was performed. Data are from (Feldhofer) Richards and Trinkaus (10), (Paviland) Richards et al. (3) and Devièse et al. (56), (Tianyan) Hu et al. (13), (Jonzac) Richards et al. (8), (Spy) Naito et al. (37), (Scladina) Bocherens et al. (2), (Goyet) Wissing et al. (7), (Marillac) Fizet et al. (1, 57), Bocherens et al. (5), (Oase) Trinkaus et al. (12), (Les Cottés) this study, (Grotte du Renne) Welker et al. (6), and (Buran Kaya III) Drucker et al. (30). Over the last few years, several studies have challenged the interpretation of high Neandertal bone collagen nitrogen isotope values as indicative of large herbivore consumption. It has been suggested that the systematic slightly higher δ15N values observed in Neandertal collagen, especially relative to that of the associated carnivores, could be explained by mammoth consumption (4), as mammoths generally exhibit higher δ15N values than other associated herbivores. Furthermore, recent studies carried on dental calculus highlight the existence of plant consumption (16⇓⇓⇓–20), challenging the interpretation of a purely carnivorous diet of Neandertals. Other factors such as the consumption of putrid meat (21), mushrooms (16, 22, 23), freshwater fish (24), or cooked food (25, 26) have also been suggested as explanations for the elevated δ15N values of Neandertal collagen. The selective hunting of young animals by Neandertals and older animals by carnivores is another factor that would increase δ15N values, but the zooarcheological data generally suggest the opposite pattern (27). Bocherens et al. (28) also suggested that the difference between Pleistocene modern humans and Neandertals would not be explained by distinctive diets, but by a climate change during the Middle to Upper Paleolithic transition causing the elevation of the N isotope signature of the local plants, impacting the values in the whole food web and, therefore, temporally distinct populations. Most of the modern humans analyzed so far are indeed from Upper Paleolithic contexts while the Neandertals (24, 28), with the exception of the Grotte du Renne (AR-14) (6) and the Spy Cave individuals (29), were found in Middle Paleolithic contexts. Finally, Drucker et al. (30) demonstrated, using compound-specific isotope analyses (CSIA), that some elevated δ15N values of Paleolithic modern human bone collagen could mainly be explained by mammoth consumption, rather than freshwater fish. The above-mentioned factors (Table 1) are all likely to increase the δ15N of body tissues without dramatically influencing the δ13C values. It is therefore very difficult to estimate which one accounts for the pattern observed in bulk Neandertal bone collagen. Novel isotope techniques, such as compound-specific isotope analyses performed on single amino acids, could help identify the factor explaining this pattern. The CSIA are a very powerful technique, which consists in establishing the C and N isotope composition of the various amino acids in bone or tooth collagen. Carbon isotope values of individual amino acids can clearly distinguish terrestrial from freshwater food sources (31, 32). Honch et al. (31) demonstrated that animals from food webs based on terrestrial plants usually exhibit similar δ13CPhe and δ13CVal (Phe: Phenylalaline, Val: Valine) in their collagen, with higher values in C4 plant-based food webs than in C3 plant-based ones. Fish consumers have δ13CVal overlapping with the animals from C3 plants (for freshwater environment) and C4 plants (for marine environment)-based food webs, but much lower δ13CPhe values (31). Nitrogen isotope values of individual amino acids reveal TP in more detail than bulk isotopic data, especially using the data obtained on phenylalanine (Phe) and glutamic acid (Glu). δ15NPhe reflects the local baseline without being really impacted by the trophic level of the animal (trophic 15N-enrichment of 0.4 ± 0.4‰) (33) whereas the glutamic acid N isotope ratio (δ15NGlu), in addition to the local baseline, is strongly impacted by the TP (trophic 15N-enrichment of 8.0 ± 1.1‰) (34⇓–36). The combination of the δ15N of these two amino acids therefore allows interpretations free of local baseline bias and allow more precisely assessing the TP of a specimen. For example, Naito et al. (37) documented the δ15N values of amino acids for the Spy Cave Neandertals (Fig. 1), revealing a possible contribution of plants into their diet, up to 20%. The use of CSIA of carbon and nitrogen can therefore potentially provide very detailed information on the diet of Neandertals and directly address the debate on the cause of the elevated nitrogen isotope signatures of their collagen. Recently, we extracted the collagen of a Neandertal tooth from les Cottés (France; SI Appendix, Supporting Information 1 and Fig. S1, and ref. 38) for radiocarbon dating (ref. 39, SI Appendix, Supporting Information 2). In the frame of the assessment of collagen preservation, we measured the C and N isotope ratios in bulk collagen and found the second highest N isotope ratio ever seen in a Neandertal sample (SI Appendix, Table S1), knowing that the first one was the above mentioned breastfeeding infant from Grotte du Renne, AR-14 (6) (Fig. 1). The δ15N value of the Les Cottés Neandertal is about ∼7‰ higher than those of the associated herbivores previously measured in Talamo et al. (40), expected one-step trophic level enrichment (9). As there are a number of suggested reasons for the high Neandertal nitrogen isotope values (we list 10 possibilities in Table 1), we aimed to investigate which ones could account for the elevated value of the Les Cottés Neandertal. The tooth root analyzed belonged to a permanent maxillary lateral incisor, and thus, the dentine formed between the fourth and eighth years of age (SI Appendix, Supporting Information 3). If explained by breastfeeding, the δ15N value associated to the Les Cottés Neandertal would indicate a very late age of weaning, which would contradict previous findings (41, 42). At Les Cottés, hyena, wolf, and fox bones with cut-marks have been found in layer 06, layer 04/upper, and layer 02. Also, mammoth ivory transformed into beads have been found into layer 04/upper (43). Carnivore and mammoth consumption are two hypotheses that have to be considered. Fish remains are not present at les Cottés. No putrid meat or mushroom consumption has been reported for this site from the archeological evidence; however, these two types of consumption would unlikely leave any significant archeological traces. Concerning the environmental hypothesis suggested by Bocherens et al. (28), the tooth was dated to 43,740–42,720 cal. y BP (SI Appendix, Supporting Information 2) and the Neandertal should thus have lived before the hypothesized increased aridity, which was suggested to have impacted local isotope signatures of the whole food webs coming from transition layers in France according to Bocherens et al. (28). We conducted N and C isotope analyses on bulk collagen and amino acids of the Les Cottés Neandertal and the associated fauna, which were identified using ZooMs (Dataset S1, Table S5), to determine which one of the abovementioned factors (Table 1) accounts for the exceptionally elevated δ15N value of the Les Cottés Neandertal. As no CSIA data are yet available for breastfeeding or suckling specimens, we also performed these analyses on (i) animal teeth from Les Cottés formed before the weaning age, and (ii) faunal and hominin remains from Grotte du Renne, including the breastfed Neandertal (AR-14). The C and N isotope data on bulk collagen of the Grotte du Renne food web are already published (6). The Grotte du Renne and Les Cottés Neandertals both have similar radiocarbon ages and come from Central France contexts, which reinforce the validity of the comparison of these two Neandertals (6, 39). Below, we report unprecedented CSIA for carbon on collagen amino acids for Paleolithic collagen samples, as well as the second CSIA application for nitrogen on collagen amino acids for Neandertals. We suggest that the exceptionally high N isotope signature values of the Les Cottés Neandertal can simply be explained by a carnivorous diet, possibly preferentially relying on reindeer (Rangifer tarandus). We also confirm that the Grotte du Renne Neandertal (AR-14) was a suckling infant and exclude the possibility of freshwater fish consumption being an influence on the collagen isotope values at both sites.
|
|
|
Post by Admin on Mar 25, 2019 18:23:21 GMT
Isotope Analyses of the Bulk Collagen. Carbon and nitrogen isotope analyses of bulk bone collagen are the classic method to estimate trophic relationship in food webs. C and N isotope analyses on the bulk collagen (δ13Cbulk and δ15Nbulk) for the AR-14 Neandertal and the associated fauna of Grotte du Renne were previously published in Welker et al. (6). The results for the Les Cottés food web are here presented. The collagen extracted from the Les Cottés samples is well preserved, with collagen yields of more than 1% and C:N ratios ranging from 3.1 to 3.3 (ref. 44 and Dataset S1, Tables S6 and S7). The herbivore bones and teeth from Les Cottés have δ13Cbulk ranging between −20.6 and −19.8‰, except for three Rangifer specimens exhibiting higher values, and the mammoth specimen exhibiting the lowest δ13Cbulk value (Fig. 2). The δ15Nbulk of the herbivores are comparable to what was observed in Grotte du Renne, except for the specimens with teeth, which would have formed during the suckling period, and that have, as expected, higher δ15Nbulk values. The mammoth specimen also has an elevated δ15Nbulk value, higher than in Grotte du Renne, but this pattern is generally observed for this species (7, 28, 30). The only carnivore species analyzed at Les Cottés beside the Neandertal is hyenas. Hyena teeth formed during or after suckling periods reveal a large range of δ15Nbulk values (9.0–11.9‰) and of δ13Cbulk values (−19.8 to −18.2‰). The hyena P3 teeth, possibly formed during the suckling period, do not show higher δ15Nbulk values than the other teeth. One of the hyenas has a δ15Nbulk comparable to that of the Rangifer teeth. The Neandertal tooth from Les Cottés falls in the range of that of the carnivore hyenas, both for C and N isotope ratios. Fig. 2. Isotope results for the site of Les Cottés. (A) C and N isotope ratios in bulk collagen. (B) N isotope ratios in phenylalanine and glutamic acid. The TP lines (B) are defined according to Chikaraishi et al. (35, 36), (C) C isotope ratios in valine and phenylalanine. The green and yellow areas (C) are defined according to Honch et al. (31). Nitrogen Isotope Analyses of Amino Acids (Les Cottés and Grotte du Renne) and Associated TP. Nitrogen isotope analyses of single amino acids were conducted to estimate the TP of hominins and associated animals, and assess which species were hunted by the different carnivores. These analyses can unravel the presence of plants, fish, meat, and suckling in the diet of the different animals. Terrestrial herbivores have a wider range of δ15NPhe values in Grotte du Renne (7.6–14.4‰) than in Les Cottés (7.3–12.4‰). The δ15NPhe values of the herbivores do not correlate with the bulk collagen δ15N values, whereas the δ15NGlu values do. This correlation, both in Grotte du Renne and Les Cottés, indicates that most of the N isotopic variability among herbivores is due to a trophic level effect, likely caused by the sampling of tissues formed during the suckling period or shortly after weaning age. The δ15NPhe value of the Neandertal in les Cottés is clearly higher than that of the other carnivores, while the δ15NPhe value of the AR-14 Neandertal falls in the range of the other carnivores (Fig. 3). The TP of all of the animals has been assessed using δ15NGlu, δ15NPhe values and a β factor of −8.4‰ (refs. 33, 36, and 37 and Dataset S1, Table S6). The β factor corresponds to an enrichment factor between the δ15N in glutamic acid and phenylalanine (δ15NGlu − δ15NPhe). This factor therefore reflects the vertical shift between each line representing a different TP in Figs. 2 and 3. The uncertainty on the estimation of the TP is probably related to the variability of the β factor, which is around 0.1 (30), and therefore yields range between 1.9 and 2.1 for herbivores and between 2.9 and 3.1 for carnivores. Several herbivores have a TP above 2.1 and one hyena equals 3.1 while the other is above this value, which corresponds to the teeth (P3, M1) and bones possibly formed during the suckling period. However, some herbivores have a TP below 1.9 (two Equidae and one Rangifer at Les Cottés, and two Equidae and one cave bear at Grotte du Renne). Moreover, one of the four hyenas of les Cottés has a TP of 2.2, similar to what is expected for a herbivore, and three carnivores at Grotte du Rennes (two Pantherinae and a wolf) have, respectively, a TP of 2.5, 2.7, and 2.8 (Figs. 2 and 3 and SI Appendix, Table S6). The Les Cottés Neandertal has a TP of 2.9, indicating a pure carnivorous diet. The Grotte du Renne Neandertal has a TP of 3.2, slightly higher than the theoretical carnivore TP and much higher than the carnivores from this site. Fig. 3. Isotope results for the site of Grotte du Renne (Arcy-sur-Cure). (A) C and N isotope ratios in bulk collagen; data from Welker et al (6). (B) N isotope ratios in phenylalanine and glutamic acid. The TP lines (B) are defined according to Chikaraishi et al. (35, 36); (C) C isotope ratios in valine and phenylalanine. The green and yellow areas (C) are defined according to Honch et al. (31). Carbon Isotope Analyses of Amino Acids (Les Cottés and Grotte du Renne). Carbon isotope analyses of amino acids can help to identify the presence of freshwater or marine resources into the diet of past hominins. The carbon isotope composition of 9–12 amino acids (depending on the derivatization method used) has been measured for all of the specimens of les Cottés and Grotte du Renne, with the exception of the Neandertal from the second site for which not enough material was available. The data are given in Dataset S1, Table S8 and fully discussed in the SI Appendix, Supporting Information 4. The δ13CPhe (−24.4 to −28.3‰) and δ13CVal (−23.2 to −28.9‰) range of values are similar at both sites and generally fit with the range observed previously in food webs relying on C3 plants (31, 32), but slightly above the average food web values reported in a third study (40). These three publications represent the totality of the existing data on δ13CPhe and δ13CVal in mammal collagen. The pattern observed between the two sites is quite different: In Grotte du Renne, the carnivores exhibit similar δ13C values in the phenylalanine and valine, while the herbivores have enriched δ13CPhe values relative to δ13CVal (Figs. 3 and 4). In Les Cottés, most herbivores have also enriched δ13CPhe relative to δ13CVal, but to a lesser extent (Figs. 2 and 4). In this case, the carnivores have depleted δ13CPhe relative to δ13CVal, especially in the case of the two hyena teeth formed during the suckling period (Fig. 4). Fig. 4. Difference (‰) between δ13Cval and δ13CPhe (Δ13CVal-Phe) in bone and tooth collagen and associated TP. The TP was estimated from the δ15NPhe and δ15NVal values. The C isotope ratios in amino acids of the Grotte du Renne Neandertal were not measured because not enough material was available. See text for more details. Neandertal Diets at Les Cottés and Grotte du Renne. Compound-specific isotope analyses conducted on amino acids provide information on the sources of dietary protein over a number of years of life with a greater level of detail compared with bulk isotope analysis. However, as these CSIA studies are in their infancy, the exact relationship of δ15N and δ13C values of amino acids to TPs is not yet fully understood. Despite these caveats, these analyses shed light on the interpretation of Neandertal diets in Europe. We demonstrate here that exceptionally high δ15N values in bulk collagen of the Les Cottés Neandertal can still be explained by a carnivorous diet solely relying on herbivore meat without any need to call upon additional explanations such as consumption of putrid meat or cooked food. These types of consumption might, however, have existed but would mean that fermentation and cooking equally impact δ15NPhe and δ15NGlu values (Table 1). Freshwater fish and carnivore meat were not—or rarely—eaten by these Neandertals. Neandertals at les Cottés could have mostly hunted reindeer from arid environments (showing elevated δ15NPhe and δ13Cbulk) or horses (also characterized by elevated δ13CAsp, SI Appendix, Supporting Information 4), whereas hyenas would have been less specific in terms of their environments and prey. This is a similar interpretation to that proposed by the only other Neandertal CSIA study by Naito et al. (37), which also suggested different ecological niches between hyenas and Neandertals. The δ13Cbulk value of the Neandertal tooth does not support here the specific hunting of mammoths, which would generally yield low δ13C (as observed in Grotte du Renne, although only one individual could be analyzed). According to CSIA analysis, the Spy Neandertals (Neandertals dated circa 41,000 BP; ref. 46) show different hunting strategies: Spy 430-a and Spy 92b hunted reindeer, horse, and bovines, while Spy 92a seemed more opportunistic (37). Neandertal hunting of reindeer, rather than mammoths, is also generally supported by zooarcheological data (27, 47) including at Grotte du Renne and Les Cottés (38, 48, 49), even if some exceptions exist for Middle Paleolithic sites (47). In Grotte du Renne, both δ15NPhe and δ13C values of AR-14 bulk collagen falls in the variability of the other carnivores. The TP of the AR-14 Neandertal is clearly above that of the other carnivores, whatever the β factor is. This finding fits with the hypothesis of a suckling infant previously suggested by Welker et al. (6). This infant was therefore not weaned but could possibly already have eaten solid food, as its TP is below 4. According to the isotope data performed on bulk collagen and amino acids, the mother was probably sharing the same ecological niche as the other carnivores. When we then look at all of the bulk collagen isotope analyses of the 29 Neandertals that have been published so far [ranging in radiometric age from ∼90,000 BP (Scladina; ref. 46) to 36,840 BP (Grotte du Renne; ref. 6)], we can see a similar and very stable diet over time. This situation could be readily explained by Neandertals being top-level carnivores consuming herbivore meat, and this pattern even continues after the arrival of modern humans in Europe (50). Our work shows that even exceptionally high δ15N bulk collagen values of two late Neandertals still result from a terrestrial meat-based diet, the same diet that was interpreted for the other 27 Neandertals that have had bulk collagen C and N isotope measurements. Despite changes in climate, environment and associated faunal spectra, throughout the Middle Paleolithic, the focus remains on large herbivores. PNAS March 12, 2019 116 (11) 4928-4933;
|
|
|
Post by Admin on Apr 25, 2019 17:44:55 GMT
Abstract Neanderthals were the only human group in Europe throughout the Late Pleistocene until the arrival of modern humans, and while their presence has been confirmed in the surrounding regions, no Neanderthal fossils are known to date from the Central Balkans. Systematic excavations of Pešturina Cave (Serbia) resulted in the discovery of a permanent right M1 (Pes-3). The specimen was recovered from stratigraphic Layer 4b with an estimated age of 102.4 ± 3.2 ka, associated with Mousterian artifacts. The exceptional state of preservation and minimal wear of the molar enabled a detailed description and comparative analysis of the inner and outer dental structure, including non-metric dental traits and morphometric features of the crown, roots, and dental tissues. The results of this study strongly support the identification of Pes-3 as Neanderthal. Non-metric traits of the occlusal surface of the crown, enamel-dentine junction, and roots are consistent with Neanderthal morphology. The crown shows morphometric features typical for Neanderthal M1, such as a buccolingually skewed crown shape, internally compressed cusps, and a relatively large hypocone. The specimen also shows Neanderthal-like dental tissue proportions, characterized by relatively thin enamel and large coronal dentine and coronal pulp volumes. The discovery of the Pes-3 molar therefore confirms the presence of Neanderthals in the territory of Serbia and the Central Balkans at the end of Marine Isotope Stage (MIS) 5c. Journal of Human Evolution Volume 131, June 2019, Pages 139-151 A single tooth may not seem like much, but a lot of information can be drawn from it. We knew it was about 100,000 years old, because the layer it was found in had previously been dated. We were able to build a high-resolution 3D model to study the shape of the crown, roots and internal structure. We made detailed measurements and performed statistical analyses which are published in the June 2019 issue of the Journal of Human Evolution. The Central Balkans could hold the key to answering these questions. Sitting at the “crossroads of Europe,” the Balkan Peninsula represents the intersection of several important migration corridors. Rivers like the Danube cut paths through mountain ranges, creating highways for migrating animals and people to follow. Modern humans followed these routes when they first migrated into Europe, funneled through the same valleys the Neanderthals called home. Pešturina Cave sits along one of these migration routes, in the side of Jelašnica Gorge, facing out towards the great floodplain of the Nišava River near the modern city of Niš. Even though no one had ever found a Neanderthal fossil in Serbia before now, we were pretty sure they lived there because we have found the remains of their culture: the so-called “Mousterian” stone tool tradition. We also know that early modern human migrants made Pešturina their home later on, because we find their unique stone tool traditions as well. This makes Pešturina Cave one of very few sites in Serbia where we know that both groups lived in the same place, albeit at different times. Unfortunately, we still don’t know very much about the early prehistory of the Central Balkans, despite the long tradition of archaeological research in the region. Twentieth-century archaeologists concentrated on early farmers, Roman palaces and Medieval fortresses. Less visible and more difficult to interpret, Palaeolithic archaeology took a back seat, until now. Filling in the gaps Led by archaeology professor Dušan Mihailović of Belgrade University and Bojana Mihailović, curator at the National Museum of Serbia, our international team of researchers has been identifying and excavating caves throughout Serbia, trying to fill the gaps in our knowledge of this important region. Along with our coauthor Predrag Radović, our role on the team is to study fossil human remains. A decade ago, in a cave not far from Pešturina named Mala Balanica, we found a human jawbone which would later be dated to about half a million years old—the oldest human fossil from the Central Balkans and one of the oldest from Europe. This jawbone did not belong to a Neanderthal, but to an older (and different) kind of human called Homo heidelbergensis. But we expect to find even older remains: Human fossils have been dated to 1.8 million years ago in Georgia and to 1.4 million years ago in Spain; the Balkan crossroads lies right in the middle. Pešturina Cave has also given up other gifts as well. In the same level as the tooth, our team found a cave bear bone with a series of parallel cut marks made by stone tools. They’re not butchery cuts, and it looks like they might have a symbolic purpose. This would be a big deal because until recently, most researchers thought symbolism and artistic expression were uniquely modern human behaviors. This attitude is shifting, since we’ve recently discovered that Neanderthals probably adorned themselves with feathers, talons and shells and even painted their caves. In a collaboration between Belgrade University and the University of Winnipeg, we have been able to offer hands-on field experience to Canadian and international students. Through this collaboration, the Central Balkans will continue to give up more and more clues about our early ancestors and their relationship with the mysterious Neanderthals.
|
|
|
Post by Admin on May 20, 2019 18:54:49 GMT
The timing and the identity of the last common ancestor (LCA) of Homo neanderthalensis and Homo sapiens (referred to as Neanderthals and modern humans hereafter) are intensely debated issues (1–5). Studies of ancient DNA (aDNA) have generally pointed to a divergence time of ca. 400 thousand years (ka) ago (6), which has found support in some quantitative studies of cranial variation (7). In addition, typically discussed evolutionary scenarios tend to assume that at least some Middle Pleistocene hominins dated to 600 to 400 ka ago, or even younger, were part of the last common ancestral species to Neanderthals and modern humans [reviewed in (8)]. Multiple anatomical studies of the fossil evidence, however, have indicated that some Middle Pleistocene European hominins, particularly those belonging to the Sima de los Huesos (SH) sample, show clear affinities with Neanderthals (9–11). After some conflicting results regarding the geological age of the SH hominins (12, 13), this collection is now securely dated to 430 ka ago (14), an age that is confirmed by the analyses of the length of its mitochondrial DNA (mtDNA) branch (15). In addition, recent analyses of the nuclear DNA (nDNA) of this population have demonstrated an evolutionary affinity of SH hominins with classic Neanderthals (16), thus making the divergence between Neanderthals and modern humans necessarily older than the age of the SH fossils. Some recent studies reflect these new findings and favor an earlier age for this LCA of 550 to 765 ka (17) based on more recent estimates of the human mutation rate (16). Divergence times inferred from genomic data are highly dependent on mutation rate and generation time estimates, which are still debated (18). Small variations of these parameters can result in very different estimates of the divergence time between two species. If these nuances are not considered, then a strict read of the values provided by aDNA analyses can give rise to radically different interpretations of the fossil record, which can even be incompatible with the affinities inferred from the anatomical evidence. The closer evolutionary affinity of SH with Neanderthals than with modern humans indicates that SH hominins diverged from the modern human lineage at the same point as classic Neanderthals did. Therefore, the genetic affinities, geological age, and morphological variation of SH hominins can be used to infer the timing of the Neanderthal–modern human divergence. Recent studies of hominin variation have demonstrated that, unlike other traits, postcanine dental shape as described through geometric morphometric datasets (fig. S1) has evolved neutrally and at extremely homogenous rates in all hominin lineages (19). This observation was used in the present study to infer the time at which Neanderthals and modern humans should have diverged to maintain the evolutionary rate for dental shape of the phylogenetic branch leading to SH hominins within the same range of variation observed in the other hominin species (tables S1 and S2). Dental shape in SH hominins is unexpectedly derived toward the Neanderthal condition, both in the expression of Neanderthal discrete features (9) and in its extreme degree of postcanine structural reduction in the number and size of cusps (fig. S2) (11). The dental shape of SH hominins is so derived that it is not representative of other Neanderthal populations. However, this does not affect the design of this study. Even if SH hominins do not show the average dental shape observed in later classic Neanderthals, their highly derived dentitions must have evolved from the same ancestral shape as classic Neanderthals did and over the period of time that separates the SH hominins from the Neanderthal–modern human LCA (see Fig. 1). The homogeneity of evolutionary rates for dental shape stands out in stark contrast with the much more heterogenous scenario observed for dental size, for which different rates are observed at different branches of the hominin phylogeny (19). Fig. 1 Phylogenetic scenarios and SH dental morphology. (A) Hominin phylogeny used in the analysis of evolutionary rates (phylogeny-1). The SH branch is represented in teal, and the LCA branch in orange, which are the colors used to represent the evolutionary rates on these two branches in Figs. 4 and 5 and fig. S5. Gray lines represent the different divergence times that have been evaluated. (B) Transformation of the Neanderthal-Denisovan-SH lineage into the SH lineage. (C) Densitree showing a randomly selected sample of 100 phylogenies [of the total sample of 60,000 phylogenies generated by Dembo and colleagues’ Bayesian analysis of hominin phylogenetic relationships (20)]. Dembo’s original trees have been pruned to preserve only the species for which dental data are available. The length of the Neanderthal branch has been shortened to reflect the age of the SH branch. (D) Upper and lower postcanine dentition of one representative SH individual (upper dentition is represented on the left). Photo credit: A. Muela, photographs taken at Institute of Health Carlos III. To account for the lack of consensus on hominin phylogenetic relationships, analyses were based on two different phylogenetic frameworks (fig. S3) (19, 20). The first one (phylogeny-1) is the phylogenetic tree used in a previous study of hominin evolutionary rates (19), which is based on the first and last appearance dates of those hominin species for which shape data for all posterior teeth were available. The second phylogeny (phylogeny-2) is the maximum clade credibility (MCC) tree calculated by Dembo and colleagues (20, 21) as part of their Bayesian analysis of hominin phylogenetic relationships. This phylogeny was pruned to include only the species for which dental data were available. In those two phylogenies, the age of the Neanderthal–modern human LCA was changed from 500 ka, which is right below the lowest bound of the interval suggested by the most recent molecular analyses (16, 17, 22), to the age of the subtending node at 100-ka intervals (Fig. 1). Uncertainty about hominin phylogenic relationships and branch lengths was explicitly addressed by estimating evolutionary rates over a sample of 100 trees. This sample of trees was randomly selected out of a sample of 60,000 trees generated through the Bayesian analysis of the hominin phylogeny (20, 21) (Fig. 1). Denisovans (23), who diverged from classic Neanderthals after the Neanderthal–modern human divergence but before the age of SH fossils (16), were not incorporated into these analyses because very scarce phenotypic data are available for this group. However, considering their evolutionary relationships (24), Denisovans, as SH hominins, can be considered part of the Neanderthal lineage in the broad sense or H. neanderthalensis sensu lato (Fig. 1). The used methodological approach consisted of a three-step process that included calculating ancestral values using a multiple variance Brownian motion (mvBM) approach (25), calculating the amount of change per branch as the difference between descendant and ancestral morphologies, and comparing these values with those obtained when simulating evolution at a constant rate across all branches of the hominin phylogeny (19). The major advantage of this approach is that it specifically and quantitatively accounts for the possibility that the LCA of Neanderthals and modern humans (or of any other two species) was not intermediate in morphology between both daughter species but more similar to Neanderthals. This is a scenario that has been recently suggested to explain the presence of derived Neanderthal features in the SH sample (4) and even in earlier European hominins (26), but that has not been formally tested yet.
|
|
|
Post by Admin on May 21, 2019 18:35:39 GMT
Changing the divergence time between the SH and the modern human branches has strong effects on the length of the SH branch and the antedating branch, as well as on their associated evolutionary rates. Very late SH–modern human divergence times result in very short lengths for the SH branch, which result, in turn, in very fast evolutionary rates for this lineage. On the contrary, too early SH–modern human divergence times result in very short lengths for the phylogenetic branch leading to their LCA, which is reflected in a very high evolutionary rate for this branch. Figure 2 shows how the evolutionary rates associated with the Neanderthal–modern human clade (those corresponding to the SH branch, to the modern human branch, and to the LCA branch) differ substantially when modifying the SH–modern human divergence time as described above. Because the timing of the Neanderthal–modern human LCA is the only one allowed to change, there is an inverse relationship between the evolutionary rate of the branch leading to SH hominins (or to Neanderthals) and the branch subtending it, such that a slower rate in the SH branch is associated with a faster rate in the subtending branch (Fig. 3 and fig. S4). Fig. 2 Branch-specific evolutionary rates obtained through the analysis of phylogeny-1. (A) Evolutionary rates obtained when setting the SH–modern human divergence time at 0.5 Ma ago. (B) Rates obtained when setting this divergence at 0.9 Ma ago, which is the scenario associated with the minimum SD of all the rates across the tree. (C) Rates obtained when setting divergence at 1.4 Ma ago. SH–modern human divergence times older than 1.4 Ma ago result in even higher rates for the branch antedating the SH–modern human separation, referred to in the following figures as the LCA branch. Evolutionary rates are provided above each branch (gray for rates that remain roughly constant in all scenarios, and black for rates associated with the Neanderthal–modern human clade, which are affected by changes in the SH–modern human divergence time). Fig. 3 Relationship between the evolutionary rate at the SH branch and at the LCA branch. Relationship observed when analyzing the first phylogenetic scenario (phylogeny-1). Evolutionary rates at both branches show an inverse and nonlinear relationship such that very high rates at the SH branch are associated with very low rates at the LCA branch and vice versa. This effect can be visualized in Fig. 2, which shows how these rates change depending on the assumed SH–modern human divergence time. The analysis of 100 phylogenies yields very few cases (3 of 100) where the SH branch shows the highest rate across the complete tree, but a majority of cases (59 of 100) where the antedating branch shows the highest rate across the tree (Fig. 4). According to these results, scenarios with a divergence time older than 0.75 million years (Ma) ago, which result in the LCA branch showing the highest evolutionary rate, are more likely than scenarios with a younger divergence time (Fig. 5). The fact that the branch leading to the SH–modern human clade tends to show the highest evolutionary rate in most phylogenies shows that dental divergence was strongest in the later stages of the evolution of the genus Homo. Fig. 4 Variation of evolutionary rates obtained through the analysis of 100 trees. (A) Densitree showing the sample of 100 randomly selected trees used in the calculations. (B) Boxplot comparing the maximum evolutionary rate (gray), the LCA rate (orange), and the SH rate (teal) in the 100 phylogenies. (C) Evolutionary rates obtained in the analysis of each of the 100 phylogenies showing the maximum rate across the tree (gray), the LCA rate (orange), and the SH rate (teal). Phylogenies in (C) are sorted according to their maximum evolutionary rate. The plot shows that the LCA rate is the maximum rate in a majority of phylogenies (59 of 100), whereas the SH rate is the maximum rate only in three phylogenies. In all other cases, the maximum rate is found in other branches (in most cases, in the P. boisei branch). The null expectation that dental shape has evolved neutrally across the complete hominin phylogeny is accepted only if the Neanderthal–modern human divergence is within the 0.7- to 1.2-Ma interval (Fig. 5A and table S3), which strongly suggests against divergence times outside this interval. The expectation of neutral dental evolution is supported by previous studies (19) and was tested against simulated scenarios reflecting genetic drift and excluding selection (27). The standard deviation (SD) of the evolutionary rates across the tree reaches its minimum value at 0.9 Ma ago, although the tree SDs are low and very similar for the 0.7- to 1.1-Ma interval. Figure 5B shows that the rates corresponding to the SH branch and the subtending branch become equal when the divergence time is set at 0.7 to 0.8 Ma ago. Divergence times that are substantially younger or older than 0.75 Ma ago result in evolutionary rates for the SH branch or for the antedating branch that are extremely far from the range of variation observed for all the other branches (Fig. 5B). The evolutionary rate at the SH branch falls within the 95% interval calculated for that branch through the analysis of 100 phylogenies only when the Neanderthal–modern human divergence time is older than 0.8 Ma ago (Fig. 5C). The 95% interval of rates for the antedating branch is very broad, so most divergence times are compatible with the values calculated for this branch (Fig. 5D). The combined result of all these analyses yields an interval of 0.8 to 1.2 Ma ago as the most likely divergence time for the SH branch and the modern human branch and, therefore, for the Neanderthal and modern human lineages. Rerunning these analyses using the MCC tree calculated by Dembo and colleagues (20) provides even older divergence times, with a minimum divergence time of 0.9 Ma ago calculated from the combination of all the analyses (fig. S5 and table S4). Assuming a Neanderthal–modern human divergence at approximately 600 ka ago, the age that the most recent molecular studies seem to point to (16, 17, 22), would have some consequences on the SH dental evolutionary rates. First, the SD of all the rates across the hominin phylogeny would show an unusually high value (although still within the obtained range), with respect to 1000 simulated neutral scenarios (P = 0.033 for phylogeny-1; see Fig. 5A and table S3). Second, assuming a divergence time of 600 ka ago would imply that the evolutionary rate at the SH branch was the highest across the hominin phylogeny (1.3 times greater than the evolutionary rate at the LCA branch). According to the analysis of 100 different phylogenies sampled from Dembo’s study (20), this scenario is not likely (Fig. 4). In addition, the evolutionary rate at the SH branch in a 600-ka divergence scenario would be 1.99, a value that is well outside the 95% interval of rates observed for the SH branch through the analysis of Dembo’s 100 trees (Fig. 5C). An evolutionary rate of 1.99 at the SH branch is lower than just one value observed in the analysis of 100 phylogenies (2.05), which is a clear outlier with respect to all rates observed at this branch (Fig. 4B). Results of the different analyses carried out in this study show that SH hominins must be separated by at least 400 ka from the Neanderthal–modern human LCA to maintain the evolutionary rate of SH hominins within the range of variation observed for other hominins. Therefore, making a ca. 600-ka divergence compatible with similar evolutionary rates between SH hominins and other hominin species would require a ca. 200 ka age for SH hominins, which is considerably younger than all the values that have been calculated for this population (12–14).
|
|