### Post by Admin on May 23, 2019 20:31:00 GMT

The 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).

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).

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).

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 large mammals [all African large mammals: 2.3 Ma (16); eastern African bovids: 1.4 Ma (17); large mammals in the Omo-Turkana Basin, Ethiopia/Kenya: 1.4 Ma (17); Australopithecus anamensis-afarensis: 1.2 Ma (7, 8)]. Results show that calculated P values are still small even when relaxing the age of earliest Homo to 2.33 Ma ago or hominin temporal durations to 2 Ma. For example, P values exceed 0.05 only when range overlap is at least 70%, and the mean and maximum P values over all possible overlap values are only 0.04 and 0.20, respectively (Eqs. 5c and 6c, and fig. S2, A and B). When both assumptions are simultaneously relaxed, P values exceed 0.05 when overlap is at least 50%, and the mean and maximum P values are 0.093 and 0.34, respectively (Eqs. 5c and 6c, and fig. S2C).

For our model, we assumed that the probability of recovering a fossil horizon from each of the A. sediba and earliest Homo temporal ranges is uniform through time. This assumption is a parsimonious one given that only one horizon each has been sampled from A. sediba (1, 2) and earliest Homo (5), and a uniform fossil recovery potential (FRP) (i.e., the probability of finding a fossil horizon) is a good approximation for the one hominin lineage where FRP has been explored, i.e., A. anamensis-afarensis (7, 8). However, we also have to consider the possibility that FRP is not uniform through time in the A. sediba and earliest Homo temporal ranges. Our P values would only be biased downward if FRP is greater in South Africa and eastern Africa during the geological time periods when A. sediba and earliest Homo are found, respectively. This is because if FRP is actually higher toward the end and beginning of the A. sediba and earliest Homo temporal ranges, respectively, then it is more likely that one will recover an A. sediba horizon that is much younger than an earliest Homo horizon, even if the former species is ancestral to the latter. Using the number of hominin fossil horizons—defined as midpoint ages of hominin-bearing members with age duplicates removed (see data file S3) (7, 8)—as a proxy for FRP, we find that FRP is not appreciably higher during the times when A. sediba and earliest Homo are found in their respective geographic regions (fig. S3). There is the possibility that FRP might be slightly higher in South Africa around the time of A. sediba (fig. S3), but even if we double the probability of recovering a fossil horizon in the last 25% of the ancestor’s (i.e., A. sediba) temporal range, the maximum P value is only 0.025 (fig. S4).

The issue of the origin of Homo is one of the thorniest questions in paleoanthropology and one that has led to myriad proposals and, sometimes, speculations (2, 3, 18, 19). Answers to the questions of how, when, and where the earliest representatives of the genus emerged are still in flux, owing especially to the dearth of fossil data from the relevant temporal range (3.0 to 2.5 Ma ago). It is therefore important to use all available lines of evidence when addressing a question as data poor as this one. While fossil remains from the 3.0- to 2.5-Ma-old interval are necessary to reasonably document the morphological patterns surrounding the origin of Homo, probabilistic methods such as the one used here are also critical for assessing the chronological evidence for proposed relationships between Homo and candidate ancestors. Hypothesized ancestor-descendant relationships must satisfy both temporal and morphological criteria (7, 20). We tested the first criterion here, and the second one has been tested elsewhere (11). A. sediba fails both benchmarks, and the most viable ancestral candidate for the genus Homo remains Australopithecus afarensis both on morphological (5) and temporal grounds (7, 8).

Science Advances 08 May 2019:

Vol. 5, no. 5, eaav9038

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).

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).

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 large mammals [all African large mammals: 2.3 Ma (16); eastern African bovids: 1.4 Ma (17); large mammals in the Omo-Turkana Basin, Ethiopia/Kenya: 1.4 Ma (17); Australopithecus anamensis-afarensis: 1.2 Ma (7, 8)]. Results show that calculated P values are still small even when relaxing the age of earliest Homo to 2.33 Ma ago or hominin temporal durations to 2 Ma. For example, P values exceed 0.05 only when range overlap is at least 70%, and the mean and maximum P values over all possible overlap values are only 0.04 and 0.20, respectively (Eqs. 5c and 6c, and fig. S2, A and B). When both assumptions are simultaneously relaxed, P values exceed 0.05 when overlap is at least 50%, and the mean and maximum P values are 0.093 and 0.34, respectively (Eqs. 5c and 6c, and fig. S2C).

For our model, we assumed that the probability of recovering a fossil horizon from each of the A. sediba and earliest Homo temporal ranges is uniform through time. This assumption is a parsimonious one given that only one horizon each has been sampled from A. sediba (1, 2) and earliest Homo (5), and a uniform fossil recovery potential (FRP) (i.e., the probability of finding a fossil horizon) is a good approximation for the one hominin lineage where FRP has been explored, i.e., A. anamensis-afarensis (7, 8). However, we also have to consider the possibility that FRP is not uniform through time in the A. sediba and earliest Homo temporal ranges. Our P values would only be biased downward if FRP is greater in South Africa and eastern Africa during the geological time periods when A. sediba and earliest Homo are found, respectively. This is because if FRP is actually higher toward the end and beginning of the A. sediba and earliest Homo temporal ranges, respectively, then it is more likely that one will recover an A. sediba horizon that is much younger than an earliest Homo horizon, even if the former species is ancestral to the latter. Using the number of hominin fossil horizons—defined as midpoint ages of hominin-bearing members with age duplicates removed (see data file S3) (7, 8)—as a proxy for FRP, we find that FRP is not appreciably higher during the times when A. sediba and earliest Homo are found in their respective geographic regions (fig. S3). There is the possibility that FRP might be slightly higher in South Africa around the time of A. sediba (fig. S3), but even if we double the probability of recovering a fossil horizon in the last 25% of the ancestor’s (i.e., A. sediba) temporal range, the maximum P value is only 0.025 (fig. S4).

The issue of the origin of Homo is one of the thorniest questions in paleoanthropology and one that has led to myriad proposals and, sometimes, speculations (2, 3, 18, 19). Answers to the questions of how, when, and where the earliest representatives of the genus emerged are still in flux, owing especially to the dearth of fossil data from the relevant temporal range (3.0 to 2.5 Ma ago). It is therefore important to use all available lines of evidence when addressing a question as data poor as this one. While fossil remains from the 3.0- to 2.5-Ma-old interval are necessary to reasonably document the morphological patterns surrounding the origin of Homo, probabilistic methods such as the one used here are also critical for assessing the chronological evidence for proposed relationships between Homo and candidate ancestors. Hypothesized ancestor-descendant relationships must satisfy both temporal and morphological criteria (7, 20). We tested the first criterion here, and the second one has been tested elsewhere (11). A. sediba fails both benchmarks, and the most viable ancestral candidate for the genus Homo remains Australopithecus afarensis both on morphological (5) and temporal grounds (7, 8).

Science Advances 08 May 2019:

Vol. 5, no. 5, eaav9038