Research ArticleANTHROPOLOGY

Temporal evidence shows Australopithecus sediba is unlikely to be the ancestor of Homo

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Science Advances  08 May 2019:
Vol. 5, no. 5, eaav9038
DOI: 10.1126/sciadv.aav9038
  • 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).

  • 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.

  • 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).

  • 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.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/5/eaav9038/DC1

    Fig. S1. Confirming our probability model results (Fig. 3) with simulations.

    Fig. S2. Same analysis as in Fig. 3 but assuming hominin temporal durations of 2 Ma, A.L. 666-1 (2.33 Ma old) represents the oldest Homo fossil, or both.

    Fig. S3. Uniform probability plots for 4- to 1-Ma-old hominin fossil horizons in South Africa and eastern Africa.

    Fig. S4. Same analysis as in Fig. 3, but the probability of sampling a fossil horizon (i.e., FRP) from the last 25% of the ancestor’s range is doubled.

    Fig. S5. Schematic illustrating how proposing an ancestor-descendant relationship based on temporal evidence does not necessarily constrain the first-discovered fossil in each species to be in the “correct” order (i.e., where the ancestor’s first-discovered fossil predates the descendant’s).

    Table S1. Previously proposed ancestor-descendant hominin species pairs (n = 28), and the year discovered and geological ages of the first-discovered specimen in each species.

    Data file S1. Hypothesized hominin ancestor–descendant species pairs.

    Data file S2. Geological ages for first-discovered specimens of hominin species.

    Data file S3. Four- to 1-Ma-old South African and eastern African hominin-bearing members and their geological ages.

    Data file S4. Dataset references.

    Data file S5. R code for analyses and creating figures.

    References (26, 27)

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Confirming our probability model results (Fig. 3) with simulations.
    • Fig. S2. Same analysis as in Fig. 3 but assuming hominin temporal durations of 2 Ma, A.L. 666-1 (2.33 Ma old) represents the oldest Homo fossil, or both.
    • Fig. S3. Uniform probability plots for 4- to 1-Ma-old hominin fossil horizons in South Africa and eastern Africa.
    • Fig. S4. Same analysis as in Fig. 3, but the probability of sampling a fossil horizon (i.e., FRP) from the last 25% of the ancestor’s range is doubled.
    • Fig. S5. Schematic illustrating how proposing an ancestor-descendant relationship based on temporal evidence does not necessarily constrain the first-discovered fossil in each species to be in the “correct” order (i.e., where the ancestor’s first-discovered fossil predates the descendant’s).
    • Table S1. Previously proposed ancestor-descendant hominin species pairs (n = 28), and the year discovered and geological ages of the first-discovered specimen in each species.
    • References (26, 27)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Data file S1 (.csv format). Hypothesized hominin ancestor–descendant species pairs.
    • Data file S2 (.csv format). Geological ages for first-discovered specimens of hominin species.
    • Data file S3 (.csv format). Four- to 1-Ma-old South African and eastern African hominin-bearing members and their geological ages.
    • Data file S4 (.pdf format). Dataset references.
    • Data file S5 (.R format). R code for analyses and creating figures.

    Files in this Data Supplement:

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