DOI: https://doi.org/10.21203/rs.3.rs-2179054/v1
Morphometric statistics (“log sem”) based on standard errors of the m-coefficient (sem) associated with general equations of the form y = mx + c have been calculated from least squares regression analyses of cranial measurements of two chimpanzee species: Pan troglodytes (currently distributed north of the Congo river) and P.paniscus (the bonobo) distributed south of that river in the Democratic Republic of Congo (DRC). A phenetic tree has been obtained from UPGMA analysis of the log sem matrix for the two species of Pan. This study focusses primarily on P. paniscus. The phenetic tree is related to locality data for individuals collected in various regions of the DRC. A few specimens attributed to P. troglodytes are “misfits” in the sense that they occur within the P. paniscus group. An explanation is provided by invoking a scenario in which there was not always a geographical barrier in the form of the Congo river, and hybridisation could occur. Genetic evidence points to at least two periods within the Pleistocene during which hybridisation took place. It is proposed that log sem values based on anatomy are indirectly sensitive to variability in gene pools. Alpha taxonomy (assuming clear boundaries between taxa) is not necessarily always possible, hence the need for a probabilistic species definition (“sigma taxonomy”), without assuming boundaries (Thackeray, 2018). These concepts are relevant to challenges associated with the taxonomy of early Pleistocene hominins, especially if there is no clear boundary between species representing Australopithecus and early Homo.
Morphometric statistics termed “log sem”, based on standard errors of the m-coefficient (sem) associated with general equations of the form y = mx + c, have previously been calculated from least squares regression analyses of cranial measurements of two chimpanzee species: Pan troglodytes (currently distributed north of the Congo river) and P.paniscus (the bonobo) distributed south of the river in the Democratic Republic of Congo (DRC) (Gordon and Wood, 2013; Thackeray and Dykes, 2016). A phenetic tree has been obtained (Thackeray, 2022) based on UPGMA analysis of a log sem matrix for specimens of P. troglodytes and P. paniscus. This study focusses primarily on P. paniscus. The phenetic tree is related to locality data for individuals collected in various regions of the DRC. A scenario needs to be explored to explain “misfits” in the tree. The concepts of hybridisation and “sigma taxonomy” can be invoked, associated with the lack of clear boundaries between taxa and episodic changes in the level of the Congo river as an intermittent geographical boundary. Analogous situations involving episodic expansion and contraction of Plio-Pleistocene habitats can be considered for hominins (Australopithecus and early Homo) in East and South Africa, separated intermittently by miombo woodland in the regions of Zambia, Malawi and Zimbabwe, associated with the lack of a clear boundary between the two hominin genera.
Crania of P. paniscus are curated by the AfricaMuseum in Tervuren, Belgium. Crania of P. troglodytes are curated by n the Powell-Cotton Museum in Kent, England.
Pairwise comparisons of cranial measurements of two chimpanzee specimens (Gordon and Wood, 2013, Supplementary Information) result in two log sem values: one in which specimen A is regressed against specimen B, and vice versa (Thackeray and Dykes, 2016). This results in a large matrix of log sem values for all comparisons. The log sem matrix is subjected to UPGMA analysis (Unweighted Pair Group Method with Arithmetic Mean) to produce a phenetic tree which can in turn be related to geographical distributions.
Part of a phenetic tree (Thackeray, 2022) based on a log sem matrix for specimens of Pan is reproduced in Fig. 1, together with locality data for individuals of P. paniscus collected in various regions of the Democratic Republic of Congo (DRC).
In general the tree for specimens of Pan (Thackeray, 2022) distinguishes the two species, but three specimens of P. troglodytes are “misfits” in the sense that they occur within the P. paniscus group.
Specimens of P. paniscus from the general locality of Kisangani (catalogued as 15293, 15294, 15295, 15296, 29050) are distributed randomly across the tree. This can be related to gene flow within the DRC region over long periods of time. Specimens from the Ubundu and Djolu localities are grouped (Fig. 1) and are considered to reflect gene flow within a radius of 300 km from Kisangi. Certain specimens situated > 300 km to the west of Kisangani, and close to or on the Congo river, cluster generally with specimens attributed to P. troglodytes. In particular, specimen M234 (P. troglodytes) groups with three specimens of P. paniscus (9338, 11353 and 20882) which are on or close to the southern bank of the Congo River.
An explanation for the “misfits” can be provided by invoking a scenario (Thackeray, 2015) in which there was not always a geographical barrier in the form of the Congo river, the level of which dropped during relatively cool palaeoclimatic conditions at the time of Pleistocene “glacial” periods in Marine Isotope Stages such as MIS 2 and MIS 6 (circa 20,000 and 160,000 years ago respectively). During such cooler conditions in West Africa (associated with lower levels of the Congo river), individuals would have crossed the river and hybridised. P.paniscus is known to be promiscuous.
Genetic evidence points to at least two periods within the Pleistocene during which hybridisation occurred (de Manuel et al, 2016). The chimpanzee “misfits” can be related to temporal and spatial dynamics in the context of palaeoclimate, episodic changes in geographical distributions, genetics and anatomical (phenetic) variability.
It is proposed that log sem values (morphometric indices based on anatomy) are indirectly sensitive to variability in gene pools.
Alpha taxonomy (assuming clear boundaries as if species can be classified distinctly) is not always possible for many taxa, hence the need for a probabilistic definition without assuming clear boundaries. This can be related to sigma taxonomy defined as follows (Thackeray, 2018): “The classification of taxa in terms of probabilities of conspecificity, without assuming distinct boundaries between species”, where sigma is the Greek letter for S (Σ) standing for the concept of a spectrum (Thackeray and Schrein, 2017).
The results presented in this study can be assessed in relation to three hypotheses proposed by Thackeray (2018):
H1: There is no clear boundary between P. troglodytes and P. paniscus. This hypothesis is supported by the results presented in Fig. 1 in this study and by Fig. 2 presented by Thackeray (2022).
H2: There is no clear boundary between certain species attributed to Australopithecus and Homo. This hypothesis is supported by results suggested in Fig. 3 presented by Thackeray (2022), based on log sem statistics.
H3: Certain hominin species attributed to Australopithecus and to Homo were capable of interbreeding. This hypothesis is relevant to the concept of a chronospecies in relation to palaeoclimatic change and the episodic expansion and contraction of habitats suitable for Australopithecus africanus in South Africa and Homo habilis in East Africa (Thackeray, 2015). Species representing Australopithecus and early Homo may have been separated intermittently by the closing of miombo woodland in regions of Zambia, Zimbabwe and Malawi. The miombo woodland can be considered to be analogous to the Congo river, as an intermittent barrier.
The three hypotheses referred to above serve to underscore the importance of developing a probabilistic definition of a species that relates to sigma taxonomy, especially in the context of the lack of clear boundaries between taxa. Notable is the fact that log sem values were used by Thackeray et al (2005) to suggest that there was not a clear boundary between Homo sapiens and neanderthals in the late Pleistocene in Eurasia. The possibility of interbreeding was invoked from log sem statistics before confirmation of this by Green et al (2010) in collaboration with Svante Pääbo, based on palaeogenomics.
Acknowledgements: I am grateful to the late Sue Dykes, Caitlin Schrein and Eddie Odes for their enthusiastic assistance, as well as to Marine Cazenave, Clément Zanolli and Peter Knox-Shaw for valuable encouragement. I thank Emmanuel Gilissen for access to catalogue data for collections in the AfricaMuseum at Tervuren in Belgium. Adam Gordon and Bernard Wood provided access to cranial measurements for two chimpanzee species. José Braga kindly provided comments on concepts expressed in this manuscript.
Ethics approval and consent to participate: Not applicable.
Availability of data and materials: Hominoid cranial measurements were published by Gordon, A.D. and Wood, B.A., 2013. Evaluating the use of pairwise dissimilarity metrics in paleoanthropology. Journal of Human Evolution 65, 465–477 (Supplementary Information).
Competing interests: The author declares that no competing interests exist.
Funding: In the context of development of the “log sem” morphometric approach (related to a probabilistic definition of a species without assuming distinct boundaries), the research was supported by the former FRD (Foundation for Research Development) and the NRF (National Research Foundation) in South Africa, since 1997.
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