The origin of the African hominoid clade is a matter of current debate, with one hypothesis proposing that chimpanzees, humans, and gorillas originated in tropical Africa, while another suggests they originated in Eurasia. Support for the latter hypothesis includes biogeographical patterns inferred from the fossil record and proposed Miocene hominoid phylogenetic relationships. The absence of fossil apes from the African Late Miocene has been used as evidence that crown hominoids were not present in Africa during this period. An alternative explanation for the paucity of these hominoids is that biases in collection and preservation have affected the African Miocene fossil record. A survey of currently known African Later Miocene sites and their faunas shows that these sites generally do not contain hominoids because of small sample sizes, poor preservation, or inappropriate habitat sampling. These preservation biases have important implications for evaluating the origins of the Homininae.
Deux hypothèses différentes ont été proposées pour rendre compte de l'origine du clade des hominoïdes africains. La première suppose que les gorilles, les chimpanzés et les hommes sont originaires d'Afrique tropicale (Fig. 1b), tandis que la seconde suggère une origine eurasiatique et une migration vers l'Afrique durant le Miocène récent (Fig. 1a). Les arguments en faveur de la seconde hypothèse reposent sur des reconstitutions phylogénétiques qui suggèrent une étroite relation de parenté entre les hominoïdes africains modernes et les hominoïdes miocènes eurasiatiques [12]. Toutefois, ces relations de parenté ne sont pas toujours corroborées par d'autres analyses phylogénétiques (e.g., [56]). Les relations phylogénétiques des hominoïdes miocènes sont actuellement mal comprises et il est difficile d'interpréter ces données avec confiance pour reconstituer l'histoire évolutive des grands singes africains modernes.
En raison de l'incertitude sur les relations au sein du groupe des hominoïdes miocènes, de nombreux spécialistes se sont tournés vers l'examen d'autres types d'indices confortant l'hypothèse d'une origine eurasiatique des grands singes africains modernes. Le problème fondamental dans l'étude de l'origine du clade africain est de savoir s'il existe, en Afrique, des candidats ancestraux potentiels. Bien sûr, le meilleur moyen de tester ceci est de rechercher davantage d'hominoïdes fossiles dans le Miocène africain moyen et récent. De tels fossiles n'ont encore pas été découverts, ce qui a conduit Begun [9–11] à suggérer qu'en fait, ils n'existaient pas. Spécifiquement, Begun [9] suggère que l'absence notable de fossiles de grands singes au Miocène récent infirme l'hypothèse d'une origine africaine à cette période. Alternativement, il propose que les ancêtres des hominoïdes aient migré vers l'Eurasie il y a 17 Ma, le rameau africain revenant en Afrique entre 6 et 9 Ma (Fig. 1A). Begun [9] dénombre, en Afrique, 26 localités fossiles datées du Miocène récent, pour lesquelles les grands singes ne sont pas représentés. Selon cet auteur, ceci constitue une démonstration suffisante pour conclure que les hominoïdes sont absents en Afrique à cette période. Le principal objectif de cette étude est d'évaluer ces assertions en examinant les données relatives à ces localités fossiles.
Les données ont été établies à partir de 25 localités africaines datées entre 12 et 5 Ma ; elles incluent les taxons mammaliens présents, le nombre de fossiles collectés, le contexte écologique supposé, les biais (taphonomie, préservation) pouvant affecter ces sites. Les résultats de cette étude montrent que les localités miocènes connues en Afrique sont inégalement distribuées dans l'espace (Fig. 2). Sur 25 localités fossiles africaines étudiées, presque toutes sont orientales (
L'hypothèse selon laquelle les hominoïdes fossiles ont pu être présents en Afrique mais n'ont pas été échantillonnés dans l'enregistrement fossile, est confortée par l'extrême rareté des fossiles de chimpanzé et de gorille dans le registre fossile. Les lignées conduisant aux chimpanzés et aux gorilles étaient vraisemblablement déjà présentes en Afrique au moment de leur divergence à partir de leur ancêtre commun, il y a environ 8 Ma. Malgré tout, leurs restes fossiles sont encore très rares avec un seul fossile probablement apparenté au gorille [55,60]. Ceci suggère que l'enregistrement fossile représentant le Miocène africain est incorrectement échantillonné, ceci étant d'autant plus vrai dans le cas des taxons décrivant un milieu forestier. Finalement, le seul moyen de préciser l'origine géographique du rameau africain des hominoïdes est de rechercher de nouveaux sites à hominoïdes sur le terrain, particulièrement dans les régions actuellement non prospectées. En identifiant de nouvelles régions d'exploration paléontologique, plus en adéquation avec les milieux de vie habituels des grands singes, nous pourrons trouver les nouveaux fossiles essentiels à la résolution du débat actuel sur la phylogénie et la paléobiologie des hominoïdes.
The African hominoid clade consists of humans, chimpanzees, gorillas, and extinct forms more closely related to them than orangutans. This group is also often referred to as the Homininae or hominines (
H1:
This hypothesis proposes that stem catarrhines and hominoids arose in Africa. Some lineages then migrated into Asia giving rise to modern gibbons, orangutans, and Eurasian Miocene apes. The stem African hominine lineage remained in Africa, eventually diversifying into gorillas and the chimpanzee-human lineage, which in turn diverged into the chimpanzee and human lineages. Thus, some crown hominoids persisted in Africa throughout the Miocene, and all modern hominoids have an ultimate African origin.
H2:
This hypothesis also suggests that the stem catarrhines originated in Africa. The common ancestor of the extant large hominoids migrated to Eurasia approximately 17 Ma. This lineage then diversified into the modern and fossil Eurasian apes, and all remaining African Miocene hominoids went extinct. The common ancestor of the Homininae arose in Eurasia in the earliest Late Miocene and migrated back to Africa between 6 and 9 Ma ago, diversifying rapidly into the modern African forms including hominins. Thus, in this hypothesis, hominines have a Eurasian origin.
The first hypothesis (H1) has been the traditional view for many years (e.g.,
New hominoid discoveries in Eurasia have caused some researchers to support the second hypothesis – that the ancestor of all living hominoids migrated out of Africa into Eurasia, and that the common ancestor of the African apes migrated back into Africa in the Late Miocene. Begun
Begun’s reasons for suggesting a Eurasian origin for the Homininae stem mostly from the fact that he, like many researchers, does not recognize any of the few known African Miocene hominoids as good ancestral candidates for modern African apes and humans. Instead, based on cladistic analyses, he believes that
Stewart and Disotell
There are many issues to consider in evaluating the two models for the origin of the Homininae. First, they are effectively mutually exclusive hypotheses – either hominines arose in Africa or they arose in Eurasia. These hypotheses have been supported using both phylogenetic and biogeographic evidence. Only by locating new hominoid fossils can these alternative hypotheses be adequately tested. For the present time, we must be content to evaluate these hypotheses based on the evidence at hand – the currently known fossil evidence on which they are based.
In general, the phylogenetic relationships of Miocene hominoids are poorly understood
It is important to note that the probability of recognizing a Eurasian fossil taxon as a likely ancestor for the Homininae is higher simply because there are more taxa known. In other words, the fact that there seem to be better modern hominoid ancestors in Eurasia could be an artifact of there being more candidates to choose from. Uncertainty in Miocene hominoid relationships has led many researchers to examine other types of evidence to strengthen the assertion that modern African apes arose in Eurasia.
It is generally recognized that there is a gap in the fossil record of African hominoids between 13 and 6 Ma ago. Traditionally, this gap has been viewed as a period of ‘missing evidence’ when hominoids are not found due to a lack of collections from suitably aged and located deposits (e.g.,
Some authors have commented that Begun’s assertion that hominoids were absent from Africa is premature because the fossil record from this period is extremely poor
One approach to this question is to look at the distribution and characteristics of currently known African Late Miocene sites in order to determine if they provide adequate evidence to conclusively determine the presence or absence of fossil apes in the early Late Miocene of Africa. Begun
Many different factors affect the composition of a fossil assemblage, in addition to the composition of the biota from which the fossil assemblage derives. Factors that can affect the probability of finding hominoids at a given site include:
Chimpanzees and gorillas have extremely similar morphologies, and have been suggested to be roughly scaled versions of the same animal
With these factors in mind, a literature review was conducted for the African localities listed by Begun Species diversity – the faunal list of all mammalian taxa reported at the locality. Sample size – the total number of fossils removed from the locality. Ecological context – as reported in the literature, or inferred from faunal lists and/or geology. Taphonomy – information on potential biases in preservation.
Once data was collected for these parameters, sites were evaluated to determine whether it is likely that hominoids would be found at the fossil site if they had been present in the living community. The prediction is that sites that contain fossil hominoids should have relatively large sample sizes and should represent forested environments. Fossil sites that do not have these characteristics should not be expected to contain fossil hominoids. If there are several sites that meet these criteria, but do not contain fossil hominoids, then this might be good evidence that hominoids were absent from Africa during this period. On the other hand, if none of the surveyed sites meet these criteria, then it is premature to assume that hominoids are indeed absent.
Often, complete information was not available for all the data categories for a given site. In particular, information on total sample sizes and the abundance of each fossil taxon at a locality are often not reported in the literature. Also, many research reports do not include the numbers of unidentifiable specimens that were recovered from the site. The total number of specimens is reported whenever it is known, but in many cases I have estimated the total sample size, dividing sites into categories of small (
An additional issue is that taxonomic identifications for many of the localities excavated in the earlier part of the twentieth century are often out of date. This was not a major problem because identifications to the genus level were adequate for the purposes of this project. Faunal lists are as thorough as possible, but it seems likely that at many sites there are additional taxa not described in the literature – particularly micromammals.
After reviewing the available information for the African localities between 5 and 12 Ma old listed by Begun
The temporal distribution of African Miocene fossil localities is also uneven (
Although hominoid remains are extremely rare in Africa between 5 and 12 Ma ago, they are present. Three of the fossil sites have hominin remains – Lothagam, the Lukeino Formation, and the Middle Awash deposits
Sites that contain either apes or hominins have larger than average sample sizes. The site with the smallest sample size that contains a hominoid is Nkondo in Uganda with a total of 832 fossils collected
Sahabi in Libya and Bled Douarah in Tunisia are both large North African fossil sites. Sahabi has an extremely diverse fauna that contains several species of carnivores and Old World Monkeys
The Nchorora Formation in Ethiopia is dated to 10.7–10.5 Ma
The Mpesida Beds in the Tugen Hills of Kenya are dated to 7–6.2 Ma. Outcrops of these beds occur in distinct, unconnected lenses of different ages
The Sinda Basin localities are found in the Western Rift Valley in Eastern Uganda
The Ibole member of the Wembere–Manonga Formation in the Manonga Valley, Tanzania is dated to 5.0–5.5 Ma by biostratigraphic correlation with other East African sites
Most fossils from Langebaanweg in South Africa have come from the ‘E’ quarry locality and the fauna has been roughly dated to 5 Ma by biostratigraphy. Thousands of specimens have been removed from this locality from several fossil horizons in the Varswater Formation
Thus none of the Late Miocene African sites surveyed would necessarily be expected to contain hominoids ancestral to modern African apes. Sites that do contain fossil apes share the following features: (1) they sample forested environments, (2) have large sample sizes with at least 500 specimens, and often considerably more, (3) have a high taxonomic diversity, (4) and a high quality of preservation. Few sites combine all of these characteristics, and those that do contain ape remains (Ngorora, Samburu, and Nkondo). None of the other fossil sites examined have all these features (
In summary, many Late Miocene African sites have extremely small sample sizes, indicating that rare faunal elements would not be preserved. Others have obvious sampling biases, containing only fragments of very large mammals, or consisting solely of micromammal teeth, and others probably did not sample suitable habitat for apes. Thus it is not unexpected that they do not contain fossil apes and these sites should not be used as evidence to suggest that hominoids were not present in Africa at this time. I conclude therefore that biases in preservation and inadequate exploration of the African continent lead to the poor Middle and Late Miocene hominoid record in Africa, not an actual absence of crown hominoids.
Because the question of where hominines originated is a controversial one, temporal gaps in the record and fragmentary fossils allow much room for interpretation, and result in alternative hypotheses concerning the evolutionary history of Miocene hominoids. I have reviewed some of the evidence associated with Begun’s
Results of my analysis show that the Late Miocene fossil record in Africa is relatively poor, particularly when compared with the Eurasian fossil record from this period. There are numerous palaeontological sites, but many of these sites contain few fossils and are biased towards either only very large or very small taxa. Four of the twenty-five sites examined contained only micromammals (rodents and insectivores), while five contained only large mammals (
Sample sizes at most of these African Miocene sites tend to be quite small. Specific data on the number of specimens recovered from a site is frequently not available, but approximate estimates of sample size can be made for most localities. Forty-four percent of the 25 localities examined had fewer than 100 specimens, and none of these contained hominoids. At the three sites in which ape fossils were recovered, sample sizes tend to be significantly larger, with at least 500 specimens recovered (
Some sites do have large sample sizes but still do not contain fossil apes. This can be explained as the result of unsuitable environmental sampling, as these sites typically represent more open country or savannah environments. As discussed previously, it is unlikely that the immediate ancestors of chimpanzees, humans, and gorillas would have lived in such open environments. There is evidence to suggest that Miocene hominoids occupied a broader ecological niche than modern apes, however still lived in largely forested environments
There is limited evidence to suggest that some African hominoids, such as
This paper has considered one approach to examining the question of where hominines originated. A future step in this research would be to attempt to ‘quantify’ the type of palaeontological site that is likely to contain hominoids. It is generally thought that apes are expected to be relatively rare components of a fossil assemblage
There has been little palaeontological research done in central and western Africa (
Another approach to assessing the quality of the African record is to look at the distribution of fossils of the modern African apes. It is reasonable to hypothesize that chimpanzees and gorillas would have diverged in Africa, even if their common ancestor had migrated to Africa from Eurasia. The recent discovery of the hominin species
It can also be hypothesized that throughout this period the chimpanzee and gorilla lineages were similar in their ecology and morphology to their descendants, being large bodied, rare, forest dwelling frugivores
There are in fact very few known chimpanzee or gorilla fossils. Reported chimpanzee fossils include associated dental remains from near Lake Naivasha, Kenya
Following restudy, none of the purported chimpanzee fossils actually is a chimpanzee. The Lake Naivasha dental remains, thought to represent a canine and two premolars, are carnivore incisors, most likely a hyaenid (L. Werdlin, J. Barry, personal communication). The Serengeti chimpanzee canine is not fossilized, but modern, is of uncertain provenience having been moved around in museum collections, and is unlikely to be from the Serengeti region (W.D. Heinrich, personal communication). The Mumba material has been restudied and represents
This reanalysis shows that fossil remains of chimpanzees and gorillas are extremely rare in the African fossil record, indeed effectively absent. This could mean that chimpanzee and gorilla remains are not frequently fossilized due to biases in preservation in heavily forested environments. However, numerous Pleistocene and Holocene archaeological sites containing faunal remains indicative of forested settings have been reported throughout Central Africa
The suggestion that hominoids ancestral to hominines lived in Eurasia and not Africa during the Late Miocene is based on the fact that appropriate ancestral candidates have not been found in Africa
A literature review of known African early Late Miocene sites demonstrates that these sites are unevenly distributed in space and time (
Many currently known Miocene fossil localities do not contain hominoids. This can be interpreted to mean either that hominoids were not present in these areas
Ultimately, the only way to know where the ancestor of the Homininae resided is to look for new hominoid sites, particularly in currently unsampled areas. Large portions of Eurasia as well as Africa are unstudied, leading to serious gaps in our knowledge of hominoid history. By identifying new areas for palaeontological exploration, particularly in areas where modern apes live, we may find new fossils that will help resolve current disputes in hominoid phylogeny and palaeobiology.
This research was supported by the Department of Anthropology at Harvard University and the American School for Prehistoric Research. This work was presented as a poster at the American Association of Physical Anthropology meetings in 2003 and has been much improved by the comments of many individuals there. David Pilbeam, John Barry, and Meg Crofoot read early drafts of this manuscript. Franck Guy provided the French translations. Larry Flynn and Michele Morgan provided advice and helped track down hard to find references. Lars Werdelin, Charles Musiba, Wolf Dieter-Heinrich, Terry Harrison, and Hansjürgen Müller-Beck helped find and identify purported chimpanzee remains. Nathan Young not only read drafts and provided many helpful comments, but also provided an almost embarrassing amount of assistance with the illustrations.
Fig. 1.
Map of Africa showing the spatial distribution of 25 fossil localities listed by Begun
Fig. 2. Carte de l'Afrique, montrant la distribution géographique des 25 localités datées entre 5 et 12 Ma et examinées par Begun
Temporal distribution of the 25 fossil localities dated to between 12–5 Ma.
Fig. 3. Distribution en fonction du temps des 25 localités fossiles datées entre 5 et 12 Ma.
Results of the survey of Late Miocene fossil localities. The sample-size column contains known numbers of specimens when possible. Estimated sample sizes have been divided into small (< 100), medium (< 1000), and large (> 1000) categories. Faunal diversity column refers to the taxonomic diversity of mammalian faunas found at the site. Environment column refers to published environmental interpretations. The inferred environment column contains personal inferences on the environment based on information from published sources. Superscript numbers refer to primary sources for the data, and are listed in the references for the paper.
Résultats de l'étude des localités fossiles du Miocène récent. La taille de l'échantillon correspond au nombre de spécimens lorsqu'il est connu. Les tailles estimées ont été divisées selon trois catégories : petit (< 100), moyen (< 1000), et grand (> 1000). La diversité faunique réfère à la diversité taxonomique des faunes mammaliennes récoltées dans un site donné. La colonne environnement représente les inférences établies à partir des informations fournies dans la littérature, mais l'information dans la colonne sur l'environnement inféré correspond à des inférences personnelles.
Data from NOW – Neogene of the Old World Database of fossil mammals http://www.helsinki.fi/science/now.