Marked nodes correspond to last common ancestors of all hominoids 1 , hominids 2 , African hominids 3 , and chimpanzees and humans 4 ; c body mass averages smaller circles and inferred primary adaptive optima larger circles for species in each regime for primates including fossils corresponding to a and b. Numbered LCAs match nodes in b.
Also noted is the adaptive optima of chimpanzees, the earliest hominins, later early hominins, and modern humans. Named taxa are outliers to their estimated optima. To test how different estimates of body mass for O.
New Interpretations of Ape and Human Ancestry
Our results Supplementary Fig. As the phylogenetic placement of D. Overall results are consistent and robust Supplementary Fig. To test how inclusion of fossil taxa where average body mass estimates are based on only a few individuals e. We also excluded the data from Miocene apes and other fossil primates because of uncertainties about their phylogenetic relatedness.
Overall results Fig. Finally, to test how using species averages for extremely sexually dimorphic species affects our findings, we used only average female body mass for the extant primates and estimated female body mass averages for our well-sampled reliably attributed hominins 3 , leading to the removal of a number of hominin taxa O. Here Supplementary Fig. It is important to note that in some lineages, female body masses may have evolved in a different manner than average body mass, as discussed below.
Alternative hypotheses for primates focused on hominoids. Colors reflect regime assignment within each figure and are not comparable between figures. We tested the relative support of our results for each iteration of our data set compared to three other alternative evolutionary hypotheses Fig. This last hypothesis assumes that the increase in body mass occurred prior to the divergence of the hylobatids and proconsuloids.
Our results suggest that the adaptive landscape shown above Fig. Half-lives provide an estimate of the time it takes before adaptation to the new selective regime is expected to be more influential than constraints from the ancestral regime, thus providing a metric quantifying the effects of phylogenetic inertia resistance or slowness in adaptation to the optima. Combined with half-lives greater than zero, divergence of individual species means from their optimum can be an indication of constraints on adaptation from any source e.
All species within these regimes appear relatively close to their optima—shown in the distance of the smaller circles species means from the larger circles body mass optima in Fig. Included extant strepsirrhine species from the families Cheirogaleidae, Daubentoniidae, Galagidae, Indriidae, Lemuridae, Lorisidae, and species of the New World Monkey group Cebidae, as well as extant taxa Aotus trivirgatus , Tarsius bancanus , and Cacajao calvus and fossil taxa Archicebus achilles Family: Archicebidae , Karanisia clarki Galagidae , Komba robustus Galagidae , Carlocebus carmenensis Pitheciidae , Nycticeboides simpsoni Lorisidae , Branisella boliviana incertae sedis , are all evolving toward an optimal body mass of around 1.
Undoubtedly, inclusion of large-bodied extinct species for some clades e. Hominids and proconsuloids are evolving to five unique optima out of the eight estimated optima for primates Fig. The largest differences between the estimated adaptive optima for a given regime and the species placed within that regime are E. Extreme optima estimates could mean that evolution in a particular lineage is not well modeled by the current OU process, which assumes a constant rate of adaptation and constant magnitude of stochastic fluctuations e.
While an optimal body mass for either hominin species and the larger Proconsul taxon slightly below or approaching that of gorillas might seem unlikely, optima are the average trait values species within a regime would reach given enough time and free of constraints These results suggest that these large-bodied hominoids would have eventually evolved even larger body masses, and overall patterns are repeated using the data set with the smaller-bodied estimates of Orrorin and Ardipithecus Supplementary Fig. The half-life for all data sets of species averages including fossils is between 0.
Notably, when using only average female body mass for the extant primates and well-sampled female body mass averages for hominins from above, the half-life increases to 6. Relatively long half-lives could suggest that female body mass may be more constrained than average species body mass, likely because of a closer link between females and ecology 46 , 47 , 48 , Another possibility is that female body mass evolution on individual branches of the phylogenetic tree adapted at different rates than the majority of other lineages see above and ref.
Support for this latter contention is suggested when comparing the estimated adaptive optima to the female averages, where some optima are quite distant from the taxa within their selective regime Supplementary Fig.
To test how including fossil data affects our results, we reran the analysis with a phylogeny and data from only extant species Supplementary Fig. Though there are now only seven hominid species, this group again shows high variation in the number of body mass regimes, evolving towards four optima, or more than half of the total number of regimes for primates. One important change is that the LCA for hominids and African hominids as well as chimpanzees and humans now shares a selective regime with Po. Another important change is the half-life is now 0. Additionally, comparing exceedingly long half-lives of the extant females-only data set to the extant species averages data set Supplementary Fig.
The results of our novel comparative phylogenetic analysis of body mass evolution in primates have large consequences for the paleobiology of hominoid and hominin origins. First, our results suggest that the LCA of chimpanzees and humans lived in an environment that favored a body mass similar to modern chimpanzees either Pa. Consistent with fossil evidence of large body sizes, our results support earlier suggestions 9 , 10 , 28 , 29 that this LCA had a body mass close to that of modern chimpanzees.
It should be noted that this regime persisted in the earliest hominins until shifting to a smaller-bodied regime near or following depending on the data set used here the origins of Au. While this reduction in the optimal average body mass could be due to a reduction in female body mass resulting from differential effects of ecological stresses 47 —such as caused by climate variability at Hadar 3.
Thus, if an increase in sexual dimorphism in Au. The origin of Au. The regime shift to larger optimal body sizes near the origins of H. Of course, this sequence of body mass evolution and the results of this analysis depends on the relationships among taxa, but at the very least there appears to be a substantial decrease in both the species average as well as average male and female body mass for hominins between 3.
It is also suggestive that the optimal body mass for the regime that contains H.
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No doubt hominin body mass was constrained and influenced by a wealth of factors, such as sexual selection, food availability and other ecological influences, tool-use, and physiological constraints that are not tested in the current model. One possible reason that may be partially supported our findings for hominins is females appear to be more heavily influenced by the environment than males because female reproductive success depends on acquiring energetic resources for birth and lactation 48 , 49 , 57 , Thus, one would expect the rate of adaptation for females to follow broad shifts in ecology—i.
Second, our results indicate that the LCA of all hominoids shared a selective regime with hylobatids and was likely the mass of a modern gibbon, arguing against the view that hylobatids are a dwarf lineage from a great ape-sized ancestor of all hominoids e.
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Larger mass apparently did not evolve until after the divergence of hylobatids, with two regime shifts to increasingly larger body mass optima prior to the LCA of hominids. While we include stem ape Pliobates cataloniae in our main analyses, our findings without Miocene ape taxa Supplementary Fig. We also note that this body mass regime is also shared by the majority of Old World Monkeys and by the distantly related New World Monkey family Atelidae, and may be the plesiomorphic ancestral condition for catarrhines.
While it was suggested that that suspensory behavior in hominoids evolved as a necessary locomotor shift coinciding with increasing body mass 15 , a gibbon-sized ancestor of all apes argues against this hypothesis—it could be that antipronogrady first evolved in a gibbon-sized early ape, further adapting in the lineage that led to hylobatids.
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An adaptive shift favoring a larger body mass could have led some early hominoids—already adapted to a rudimentary form of suspensory locomotion—to adapt their morphology and behavior to deal with this change, leading to some of the differences between great ape and gibbon locomotor behavior In this model, there is no need for the independent acquisition of suspensory behavior among the hominoid lineages—the series of morphological changes that allow for suspensory behavior evolved once and the combination of continued use and possibly phylogenetic inertia resistance or slowness in adaptation in these characters led to their persistence while body mass appears to be extremely evolvable in this clade.
Taking a step back, suspensory behavior and increased body mass have been argued to be hominoid adaptations to a foraging strategy allowing them to compete with increasingly numerous old world monkeys since the Middle Miocene reviewed in ref. Our results suggest that these two adaptations occurred independently of each other and could have been part of an arms race with monkeys for fruit resources—suspensory behavior to access ripe fruit on compliant branches at the edges of foliage evolved first, followed by larger body sizes when direct physical competition was required.
Sexual selection in hominids likely further increased optimal average body sizes. We also note that although hominin and hominoid evolution is the focus of this analysis, our complete results suggest that the basal euprimate lived in a selective regime that favored an optimal body mass between 1. Finally, our results provide evidence of a complex and changing adaptive landscape in the hominin and hominid clades—while almost all other primates are evolving toward two body mass optima in our sample e.
While these results are preliminary, they suggest that most of primate evolution has taken place within a small number of ecological niches—one small-bodied regime principally based around arboreal quadrupedalism and leaping, one larger bodied regime, members of which evolved toward suspensory behavior Hylobatidae and Atelidae and continued arboreal quadrupedalism and leaping most Cercopithecidae.
Larger-bodied species adapted to terrestrial locomotion— Pa. Within each group is variation in locomotor behavior, as well as diet, social structure, and so on—differing local selective pressures that likely led to the variation around the optimal body mass within a given regime. Together, the greater number and greater complexity of body mass optima for hominids and hominins supports the hypothesis that dramatic and uncommon shifts in the adaptive landscape drove human evolution.
Extant primate body mass averages were taken from Isler et al. Early fossil hominin body mass species averages were taken from Grabowski et al. Body mass of Ar. We used a similar approach for the body mass of O. We used a composite phylogeny based on the dated consensus tree from version 3 of 10Ktrees 64 , the latest phylogeny for fossil hominins from Dembo et al.
Both species of Ekembo were formerly placed in Proconsul but united into their own genus based on morphology Here Ekembo and Proconsul form two clades 66 and diverge separately from the main trunk prior to the divergence of hylobatids. Proconsul and by extension Ekembo is accepted as a stem hominoid by the majority of researchers 67 although some argue that it is a stem catarrhine We placed all five early Miocene apes as originating shortly before the divergence of hylobatids, later Miocene great ape Dryopithecus originating before the Gorilla lineage, followed by Hispanopithecus , with Sivapithecus on the branch that led to Pongo.
We also updated the divergence date for H. As most of these non-hominoid fossil taxa averages are based on rare single individuals, we assumed that average body size and female body size were the same for our females-only analyses above. See Supplementary Fig.