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AbstractUnderstanding the emergence of the genus Homo is a pressing problem in the study of human origins. Australopithecus sediba has recently been proposed as the ancestral species of Homo, although it postdates earliest Homo by 800,000 years. Here, we use probability models to demonstrate that observing an ancestor’s fossil horizon that is at least 800,000 years younger than the descendant’s fossil horizon is unlikely (about 0.09% on average). We corroborate these results by searching the literature and finding that within pairs of purported hominin ancestor–descendant species, in only one case did the first-discovered fossil in the ancestor postdate that from the descendant, and the age difference between these fossils was much less than the difference observed between A. sediba and earliest Homo. Together, these results suggest it is highly unlikely that A. sediba is ancestral to Homo, and the most viable candidate ancestral species remains Australopithecus afarensis.INTRODUCTIONUnderstanding the origin of the genus Homo is one of paleoanthropology’s most enduring questions. A key element in resolving this question is determining which species may have been ancestral to our genus. Because Australopithecus sediba has recently been proposed as a candidate ancestral species (1–3), it is essential that we critically evaluate this claim. Fossil specimens from A. sediba are currently only known from Malapa, South Africa, which is dated to 1.977 million years (Ma) ago (2). These fossils postdate by 800,000 years (0.8 Ma) the only known specimen from the oldest, and currently unnamed, species of Homo (hereafter, “earliest Homo”), which is dated to 2.8 to 2.75 Ma ago at Ledi-Geraru, Ethiopia (4, 5). Most recently, the argument for A. sediba being ancestral to Homo was continued by Robinson et al. (6), who discussed how a fossil horizon from an ancestral species could be much younger than a horizon from the descendant and claimed, “On temporal grounds alone one cannot dismiss the possibility that A. sediba could be ancestral to the genus Homo” (p. 1).Two conditions must both be met for A. sediba to be ancestral to Homo and for the recovery of an A. sediba fossil horizon that is much younger than an earliest Homo horizon (barring severe postdepositional stratigraphic mixing or errors in taxonomic assignment or dating): (i) Because an ancestor’s fossil horizon can only postdate a descendant’s if there is some overlap in the species’ temporal ranges (Fig. 1A), the descendant must have speciated from the ancestor via budding cladogenesis (Fig. 1B). For our study, we assume that Homo cladogenetically budded from A. sediba because, otherwise, this analysis would be unnecessary, and the argument for A. sediba being ancestral to Homo would be illogical (because the A. sediba fossil horizon postdates the earliest Homo horizon). (ii) Given the large amount of time separating the fossil horizons of A. sediba and earliest Homo, there must have been substantial overlap between the two species’ temporal ranges, such that the end of the A. sediba range is able to postdate the beginning of the earliest Homo range by at least 0.8 Ma (Fig. 2). If range overlap is less than 0.8 Ma, then the A. sediba fossil horizon cannot be 0.8 Ma younger than the earliest Homo horizon (assuming A. sediba was ancestral to Homo) (Figs. 1A and 2). As range overlap increases, so does the probability of sampling the end and beginning of the A. sediba and earliest Homo ranges, respectively, such that their horizons are at least 0.8 Ma apart (Fig. 2A). This second condition forms the theoretical basis for our probability model. 25).” data-hide-link-title=”0″ data-icon-position=”” href=”https://advances.sciencemag.org/content/advances/5/5/eaav9038/F1.large.jpg?width=800&height=600&carousel=1″ rel=”gallery-fragment-images-1849370379″ title=”Conditions where an ancestor’s fossil horizon can be younger than the descendant’s. For both figures, “A” represents the ancestral species, and “D” represents the descendant species. (A) When there is no overlap between the temporal ranges of an ancestor and a descendant, an ancestor’s fossil horizon can never be younger than the descendant’s. If ranges partially overlap (gray), then an ancestor’s fossil horizon can postdate the descendant’s (fossil horizons are represented by white circles). The maximum age difference between a younger horizon from an ancestor and an older horizon from a descendant is ultimately constrained by the amount of range overlap, such that the age difference can never be greater than the amount of overlap. (B) Three different ways a descendant can speciate from an ancestor. Budding cladogenesis is the only speciation mode that produces ancestors and descendants with overlapping temporal ranges and is therefore the only mode where an ancestor’s fossil horizon can postdate the descendant’s. “D1” and “D2” represent two sister lineages, which are both descendants of “A.” (B) is modified after Fig. 1 in (25).”>Fig. 1 Conditions where an ancestor’s fossil horizon can be younger than the descendant’s.For both figures, “A” represents the ancestral species, and “D” represents the descendant species. (A) When there is no overlap between the temporal ranges of an ancestor and a descendant, an ancestor’s fossil horizon can never be younger than the descendant’s. If ranges partially overlap (gray), then an ancestor’s fossil horizon can postdate the descendant’s (fossil horizons are represented by white circles). The maximum age difference between a younger horizon from an ancestor and an older horizon from a descendant is ultimately constrained by the amount of range overlap, such that the age difference can never be greater than the amount of overlap. (B) Three different ways a descendant can speciate from an ancestor. Budding cladogenesis is the only speciation mode that produces ancestors and descendants with overlapping temporal ranges and is therefore the only mode where an ancestor’s fossil horizon can postdate the descendant’s. “D1” and “D2” represent two sister lineages, which are both descendants of “A.” (B) is modified after Fig. 1 in (25). Eq. 5c): Td represents the age difference of interest (i.e., 0.8 Ma), To represents the amount of range overlap, and TR represents the duration of the entire temporal range (i.e., 0.97 Ma). (B) Focusing on the black regions, a descendant’s fossil horizon (white circles) can sample some time near the species’ age of origination (leftmost example), which means that the ancestor’s horizon can be sampled anywhere in its own black region and still be at least 0.8 Ma younger than the descendant horizon (white-striped region). If the descendant’s horizon is found in the middle of the black region (middle example), the ancestor’s horizon must sample the younger half of its own black region. If the descendant horizon samples the end of its black region (rightmost example), the ancestor’s horizon must sample the end of its temporal range. The rightmost example is used to illustrate the XA and XD variables (Eq. 3), each of which represents the distance from the beginning of the black region to the temporal location of the fossil horizon in the ancestor’s and descendant’s range, respectively. For the ancestor’s horizon to postdate the descendant’s by at least 0.8 Ma, XA must be greater than XD, and two iterations of this are shown.” data-hide-link-title=”0″ data-icon-position=”” href=”https://advances.sciencemag.org/content/advances/5/5/eaav9038/F2.large.jpg?width=800&height=600&carousel=1″ rel=”gallery-fragment-images-1849370379″ title=”Schematic used to derive the model for quantifying the probability that an ancestor’s fossil horizon postdates the descendant’s by at least 0.8 Ma. For both figures, “A” represents the ancestral species, and “D” represents the descendant species. (A). The probability of sampling an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s is ultimately a function of the amount of overlap between both species’ temporal ranges relative to the age difference of interest (which here is 0.8 Ma). When range overlap is less than 0.8 Ma, the ancestor’s horizon cannot postdate the descendant’s by 0.8 Ma (represented by the Xs in the leftmost example). In the middle example, there is enough range overlap where 0.8 Ma separates the end and beginning of the ancestor’s and descendant’s ranges, respectively (black), and each species’ fossil horizon must be sampled from these black regions. As overlap increases (rightmost example), so does the size of the black regions and the probability of sampling an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s horizon. The rightmost example is used to illustrate the three variables from our probability model (Eq. 5c): Td represents the age difference of interest (i.e., 0.8 Ma), To represents the amount of range overlap, and TR represents the duration of the entire temporal range (i.e., 0.97 Ma). (B) Focusing on the black regions, a descendant’s fossil horizon (white circles) can sample some time near the species’ age of origination (leftmost example), which means that the ancestor’s horizon can be sampled anywhere in its own black region and still be at least 0.8 Ma younger than the descendant horizon (white-striped region). If the descendant’s horizon is found in the middle of the black region (middle example), the ancestor’s horizon must sample the younger half of its own black region. If the descendant horizon samples the end of its black region (rightmost example), the ancestor’s horizon must sample the end of its temporal range. The rightmost example is used to illustrate the XA and XD variables (Eq. 3), each of which represents the distance from the beginning of the black region to the temporal location of the fossil horizon in the ancestor’s and descendant’s range, respectively. For the ancestor’s horizon to postdate the descendant’s by at least 0.8 Ma, XA must be greater than XD, and two iterations of this are shown.”>Fig. 2 Schematic used to derive the model for quantifying the probability that an ancestor’s fossil horizon postdates the descendant’s by at least 0.8 Ma.For both figures, “A” represents the ancestral species, and “D” represents the descendant species. (A). The probability of sampling an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s is ultimately a function of the amount of overlap between both species’ temporal ranges relative to the age difference of interest (which here is 0.8 Ma). When range overlap is less than 0.8 Ma, the ancestor’s horizon cannot postdate the descendant’s by 0.8 Ma (represented by the Xs in the leftmost example). In the middle example, there is enough range overlap where 0.8 Ma separates the end and beginning of the ancestor’s and descendant’s ranges, respectively (black), and each species’ fossil horizon must be sampled from these black regions. As overlap increases (rightmost example), so does the size of the black regions and the probability of sampling an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s horizon. The rightmost example is used to illustrate the three variables from our probability model (Eq. 5c): Td represents the age difference of interest (i.e., 0.8 Ma), To represents the amount of range overlap, and TR represents the duration of the entire temporal range (i.e., 0.97 Ma). (B) Focusing on the black regions, a descendant’s fossil horizon (white circles) can sample some time near the species’ age of origination (leftmost example), which means that the ancestor’s horizon can be sampled anywhere in its own black region and still be at least 0.8 Ma younger than the descendant horizon (white-striped region). If the descendant’s horizon is found in the middle of the black region (middle example), the ancestor’s horizon must sample the younger half of its own black region. If the descendant horizon samples the end of its black region (rightmost example), the ancestor’s horizon must sample the end of its temporal range. The rightmost example is used to illustrate the XA and XD variables (Eq. 3), each of which represents the distance from the beginning of the black region to the temporal location of the fossil horizon in the ancestor’s and descendant’s range, respectively. For the ancestor’s horizon to postdate the descendant’s by at least 0.8 Ma, XA must be greater than XD, and two iterations of this are shown. While Robinson et al. (6) are correct that it is possible for an ancestor’s fossil horizon to be much younger than the descendant’s, a more informative question would be, “How likely is this chronological pattern?” We build upon previous work concerning the evolutionary relationships of A. sediba (1–3, 6) and construct a probability model, which serves as a null hypothesis test, to evaluate whether A. sediba is ancestral to Homo. We assume that (i) A. sediba and earliest Homo each had temporal ranges of 0.97 Ma (6), (ii) the probability of recovering fossils throughout each species’ range is uniform through time (7, 8), and (iii) the probability of sampling an A. sediba fossil horizon does not affect the probability of sampling an earliest Homo fossil horizon, i.e., these are independent events (see Materials and Methods). From these assumptions, we quantify the probability of finding one fossil horizon from A. sediba that is at least 0.8 Ma younger than one horizon from earliest Homo (i.e., the observed data), assuming A. sediba is ancestral to Homo (i.e., the null hypothesis). The computed probabilities are equivalent to P values, and if they are exceptionally low, this would suggest that A. sediba is unlikely to be the ancestor of Homo (i.e., the null hypothesis is falsified). We calculate multiple P values as a function of the overlap between the two species’ true temporal ranges, which is currently unknown. We analyze temporal evidence only (6) and do not consider morphological data concerning the evolutionary relationship between A. sediba and Homo (1, 9–11). We also analyze the historical record of hominin discovery and calculate the geological age difference between initial fossil discoveries in purported ancestor and descendant species. The aim here is to corroborate our theoretical probability results and to empirically assess how likely it is for an ancestor’s fossil horizon to postdate a descendant’s by at least 0.8 Ma.RESULTSThe probability of finding an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s is, by definition, zero when temporal range overlap is less than or equal to 0.8 Ma (Figs. 2A and 3 and Eq. 5c). This probability monotonically increases with range overlap when overlap is greater than 0.8 Ma (Fig. 3 and Eq. 5c) for reasons discussed above (Fig. 2A). However, even when the two species’ ranges completely overlap, which is impossible for ancestor-descendant species and is only presented as a theoretical upper bound, the computed P value is only 0.016 (Fig. 3 and Eq. 5c). If we treat all possible values of range overlap as equally likely, the mean P value over all overlap values is 0.0009 (Eq. 6c). We have confirmed our probability model results with simulations (fig. S1 and data file S5). 6).” data-hide-link-title=”0″ data-icon-position=”” href=”https://advances.sciencemag.org/content/advances/5/5/eaav9038/F3.large.jpg?width=800&height=600&carousel=1″ rel=”gallery-fragment-images-1849370379″ title=”Probability of finding an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s fossil horizon (P value). P values are plotted as a function of the overlap between the two species’ true, unknown temporal ranges, each of which is assumed to be 0.97 Ma in duration (6).”>Fig. 3 Probability of finding an ancestor’s fossil horizon that is at least 0.8 Ma younger than the descendant’s fossil horizon (P value).P values are plotted as a function of the overlap between the two species’ true, unknown temporal ranges, each of which is assumed to be 0.97 Ma in duration (6). Reviewing the paleoanthropology literature, we recorded 28 hypothesized ancestor-descendant species pairs (table S1). There is only one instance where an ancestor’s first-discovered fossil postdated the descendant’s: ancestor Homo erectus sensu lato (Kedung Brubus 1) dated to 0.8 to 0.7 Ma ago (12) and descendant Homo antecessor (ATD6-1) dated to 0.9 to 0.8 Ma ago (13). The age difference between these specimens (i.e., 0.1 Ma) is far less than the age difference observed between A. sediba and earliest Homo (i.e., 0.8 Ma) (Fig. 4). When the mean and SD of the 28 observed age differences are used to generate a normal distribution model (bell curve in Fig. 4), 0.8 Ma falls in the>99.9th percentile, which translates to a P value less than 0.001. Fig. 4 Histogram of the geological age differences between first-discovered fossils in purported hominin ancestor-descendant species pairs (n=28).Negative age differences represent those species pairs where the ancestor’s first-discovered fossil is older than the descendant’s, and positive age differences indicate the opposite. The black arrow represents the observed age difference between A. sediba (hypothesized ancestor) and earliest Homo at Ledi-Geraru (hypothesized descendant). The bell curve represents a normal distribution model, generated using the sample mean and SD of the 28 observed age differences. DISCUSSIONWe have demonstrated using probability models that the null hypothesis of A. sediba being ancestral to Homo can be falsified. That is, it is very unlikely (about 0.09% on average) to find an A. sediba fossil horizon that is at least 0.8 Ma younger than an earliest Homo horizon, if the former species is actually ancestral to the latter. The prior record of paleoanthropological discoveries also reflects the rarity of cases in which this chronological pattern is observed, further supporting that A. sediba is unlikely to be ancestral to Homo.We can explore how strongly our assumptions influenced our modeling results. We calculated our P values, assuming the 2.8-Ma-old Ledi-Geraru mandible actually belongs to Homo (5). Some researchers dispute this (14), so we also ran our analyses assuming A.L. 666-1 (2.33 Ma-old)—a specimen widely regarded as Homo—represents the oldest Homo specimen (15). Although a handful of researchers argue that all pre–1.9-Ma-old specimens assigned to Homo are invalidly named or are poorly dated (2, 3), we view this assertion as unlikely [as does Robinson et al. (6)]. By selecting a younger fossil to represent the oldest Homo specimen, we are decreasing the observed age difference between A. sediba and earliest Homo, which should increase the P values overall (Eq. 5c). We also explored whether our choice of 0.97 Ma to represent hominin temporal durations might affect our results because using a longer duration will increase the amount of time associated with a given percentage of range overlap between two species, and this should increase P values as well (Eq. 5c). We therefore reran our analyses assuming hominin temporal durations of 2 Ma, which is at the larger end of estimated mean species durations in African

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