Early Holocene survival of megafauna in South America

Abstract

Comments on Steadman, D.W., Martin, P.S., MacPhee, R.D.E., Jull, A.J.T., McDonald, H.G., Woods, C.A., Iturralde-Vinent, M. & Hodgins, G.W.L. (2005) Asynchronous extinction of late Quaternary sloths on continents and islands. Proceedings of the National Academy of Sciences USA, 102, 11763–11768. The debate over the causes of the Pleistocene megafaunal extinction dates back to the early 19th century (Grayson, 1984), and continues to generate considerable controversy (e.g. Grayson & Meltzer, 2003; Araujo et al., 2004; De Vivo & Carmignotto, 2004; Fiedel & Haynes, 2004; Burney & Flannery, 2005; Wroe et al., 2006). Typically, protagonists in this debate can be classified into two groups. One group argues that Late Pleistocene megafaunal extinctions were primarily caused by direct and indirect human action through hunting, habitat modification or introduction of new predators (Burney & Flannery, 2005, 2006; Barnosky et al., 2004; Fiedel & Haynes, 2004). The other interpretation is that humans had at most a minor role in the megafaunal extinction, and that the loss was attributable principally to a climatic cause (Ficcarelli et al., 2003; Grayson & Meltzer, 2003, 2004; Barnosky et al., 2004; De Vivo & Carmignotto, 2004; Boeskorov, 2006; Guthrie, 2006; Wroe et al., 2006; Wroe & Field, 2006). Here we contest the position of Steadman et al. (2005), who favour the overkill hypothesis to explain the ground sloth extinction in the Americas. Although making an important contribution to the debate on extinction of the New World megafauna, Steadman et al. (2005) make some important assumptions in their analysis. Steadman et al. (2005) argue that the extinction of ground sloths in the New World was concomitant with, and a consequence of, the human occupation of the Americas. Their argument is two-fold. First, the radiocarbon dates (14C) accepted by them for the last appearance dates (LADs) of these animals roughly correspond to megafaunal extinction dates in South and North America and the West Indies. These dates coincide with the human colonization of these regions and they argue that this supports the thesis that human arrival caused extinction of the ground sloth. Second, according to Steadman et al., extinctions caused by climatic fluctuation would result in concomitant LADs across the entire continent and associated islands, as they viewed these fluctuations as being widespread and uniform, whilst they found that the LADs for the West Indies, around 4400 14C yr bp [c. 4800–5050 calibrated years before present (cal. bp); dates calibrated with calib 5.0, Stuiver et al., 2005], are much younger than those found in the continent (c. 11,000 14C yr bp; c. 12,880–12,950 cal. bp for North America and c. 10,500 14C yr bp; c. 12,390–12,640 cal. bp for South America). We contend that the chronological data presented by Steadman et al. (2005) are incomplete, especially when considering South America. While Steadman et al. (2005) suggest that there are no acceptable Holocene LADs for ground sloths, a large number of Holocene dates generated through direct dating of bone and dung remains are indeed available in the literature. Barnosky et al. (2004; supporting material) revised the radiocarbon dates available for megafaunal remains throughout the world. In South America, they listed four articles with remains of megafauna dated within the Holocene, based both on direct and indirect dates. Even when considering only the results based on direct dates of bone remains, sufficient evidence still supports Holocene LADs for subequatorial ground sloths. For instance, from Argentina, Borrero et al. (1998) presented a total of seven 14C dates consistent with a Holocene survival of megafauna, albeit two of these ages are potentially unreliable, and four were obtained from one single specimen (indeed, one of the unreliable dates comes from this specimen; Table 1). Other reports not included in Barnosky et al. (2004) provide two direct radiocarbon ages of megafaunal bone remains from central Brazil at the Pleistocene/Holocene boundary (Table 1; Neves & Piló, 2003; Araujo et al., 2004). Politis et al. (2004; also not included in Barnosky et al., 2004) presented two additional Holocene direct radiocarbon ages of Megatherium americanum (Blumenbach) specimens (Table 1) and a third one from the Holocene/Pleistocene boundary (10,190 ± 120 14C yr bp; c. 11,820–12,020 cal. bp; Table 1), all in Argentina; and Marshall et al. (1984; also not included in Barnosky et al., 2004) reported a single Holocene age of 8910 ± 200 14C yr bp (c. 9780–10,150 cal. bp; GIF-4116) of a Scelidodon chiliensis (Lydekker) in Peru (Marshall et al., 1984;Pujos & Salas, 2004). Four of the sites where these dates were obtained are located in Argentina, while two are located in central Brazil and the last in Peru (Fig. 1). All the Argentinean sites (Arroyo Seco 2, La Moderna, Campo Laborde and Paso Otero 5) are open-air archaeological sites, i.e. the megafaunal remains are associated with prehistoric human occupations (see Borrero et al., 1998; Politis et al., 2004 for detailed descriptions). Arroyo Seco 2 is interpreted as a base camp where a large variety of activities were undertaken (Politis et al., 2004), including the exploitation of ground sloths and other megafauna by humans. However, Borrero et al. (1998) and Politis et al. (2004) do not state clearly if the two specimens (M. americanum and Equus neogeus Lund) that dated to the Holocene (Table 1) showed marks of human manipulation. The remaining open-air sites are believed to be sites used for specific activities (Politis et al., 2004): La Moderna is interpreted as an occasional megafaunal processing site, where the remains of a single glyptodont (Doedicurus clavicaudatus Owen) dated to the Holocene (Table 1) were recovered; Campo Laborde presents evidence that it was used as a hunting and processing site for ground sloths (M. americanum; Table 1); and Paso Otero 5, was also identified as a hunting and processing site for local megafauna. Archaeological and palaeontological sites in South America presenting direct Late Pleistocene/Early Holocene radiocarbon (14C) dates for megafaunal remains. Circles represent sites with no evidence of human exploitation of the megafaunal remains, whereas triangles represent sites with evidence of human exploitation of megafauna. 1, Gruta Cuvieri; 2, Escrivânia 5; 3, Gruta del Indio; 4, La Moderna; 5, Campo Laborde; 6, Arroyo Seco 2; 7, Paso Otero 5; 8, Pampa de los Fósiles. The two Brazilian sites, in contrast, are exclusively palaeontological, i.e. they are not associated with human occupations, and are located in limestone caves in the karstic region of Lagoa Santa. Gruta Cuvieri is a cave where three vertical chambers functioned as natural traps for the now extinct megafauna and other animals. The only megafauna species found so far is Catonyx cuvieri (Lund), a medium-sized ground sloth. The Holocene date presented in Table 1 was obtained from one of these ground sloths, found at the surface of one of the chambers. The other Brazilian site, Escrivânia 5, is part of a complex of caves, generically referred to as Escrivânia, representing one of the richest palaeontological limestone outcrops known at Lagoa Santa. Together with tons of animal fossil bones, in one of the chambers (Escrivânia 3) an almost complete human skeleton was also recently recovered, dated to 7650 ± 80 14C yr bp (c. 8370–8420 cal. bp; Beta 174734). The Peruvian site, Pampa de los Fósiles, is also a palaeontological site located in the Cupisnique Desert. Several archaeological sites in the region have revealed no evidence of human interaction with the megafauna in the region (Pujos & Salas, 2004). In addition to these reported dates, Steadman et al. (2005; supporting material) disqualified two other Holocene dates as unreliable (they also rejected a third date, but it has a very large margin of error). These were the only Holocene dates found in their bibliographical revision and they ‘have means that are up to 1000 years younger than means of any [of the accepted LADs] [Supplementary online material]’. As 10 reliable Holocene direct radiocarbon dates for megafauna are described here, there is no further reason to reject the dates of 8990 ± 90 14C yr bp (c. 9920–10,190 cal. bp; LP-925; Garcia, 2003) and 9560 ± 90 14C yr bp (c. 10,680–10,860 cal. bp; GrN-5772; Long et al., 1998) as unacceptable outliers. These two dates are from an Argentinean site, Gruta del Indio (Fig. 1; see Long et al., 1998; Garcia, 2003 for detailed descriptions). This site is a rockshelter, and although it presents chronological information placing humans together with megafauna in time, there is no evidence of humans exploiting the local megafauna (Long et al., 1998; Garcia, 2003). As presented in Table 1, from the 14 existing Holocene dates we found for megafaunal remains in South America eight are derived from ground sloths, which severely weakens the position of Steadman et al. (2005), that there are no acceptable Holocene LADs for ground sloths in the Americas. Assuming that human groups already inhabited South America around 12,500 14C yr bp (c. 14,300–14,950 cal. bp; Dillehay, 2000), the argument that the ground sloth LADs were concomitant with the human arrival in the New World can no longer be accepted, at least not as an immediate phenomenon. The second argument presented by Steadman et al. (2005) is that the apparent delay observed in the LADs of Central America islands, when compared with the continental ones, favours the overkill hypothesis. Delayed LADs in insular regions have been found in other parts of the world, independent of human presence (Guthrie, 2004; Boeskorov, 2006). Boeskorov (2006) showed that in northern Eurasian islands, megafauna survived into the Holocene, e.g. the mammoths of Wrangel Island. Nonetheless, the extinction of megafauna in Eurasia as a whole is believed to be primarily due to climatic changes (Barnosky et al., 2004; Boeskorov, 2006), particularly because no human presence is found in the Wrangel Islands until well after the extinction of the megafauna (Boeskorov, 2006). Although these data do not peremptorily disqualify Steadman’s argument, they do bring into question whether the overkill hypothesis is the most parsimonious explanation for megafaunal extinctions. Finally, it must be emphasized that there is a general lack of evidence of sloth remains in archaeological contexts in the Americas as a whole (but see Politis et al., 2004 for an exception), which also speaks against the overkill hypothesis. Specifically, in Lagoa Santa, despite the excavation of dozens of archaeological sites dated to the Pleistocene/Holocene transition (showing human evidence as old as 11,000–11,500 14C yr bp; c. 12,880–13,400 cal. bp; Neves et al., 1999), evidence is lacking of megafaunal use by humans, either as a source of food or raw material (Kipnis, 1998; Prous & Fogaça, 1999). In North America, a similar situation is observed. According to Grayson & Meltzer (2003), there are only two genera of megafauna (Mammuthus Burnett, 1830 and Mammut Blumenbach, 1799) known to have been hunted by humans during the Clovis period (Grayson & Meltzer, 2003). This scenario is accepted even by Fiedel & Haynes (2004), strong defenders of the overkill hypothesis. Thus, at least in South America (and most probably in North and Central America as well), the idea that ground sloths went extinct due to overkill lacks archaeological support. In conclusion, the ground sloth overkill hypothesis, as defended by Steadman et al. (2005), is not sufficiently supported in the empirical world. As we have briefly pointed out: (1) a considerable number of reliable Holocene dates for megafaunal specimens in South America already exist, including for ground sloths; (2) the existence of late megafaunal LADs in Central America islands can be equally well explained through overkilling or environmental changes; and (3) the general lack of megafaunal killing sites and megafaunal remains in archaeological contexts is inconsistent with the overkill hypothesis. Nonetheless, it is important to emphasize that the amount of information regarding the presence of megafauna in archaeological sites is still too small to be considered as strong evidence against human predation of megafauna, and thus this piece of information must be interpreted as complementary to the others. Collectively, the data presented here are more consistent with a model explaining megafaunal extinction through climatic fluctuations, although in South America the poor chronological contextualization of the megafaunal decline does not yet allow for a percentage estimate of megafaunal genera that survived until human arrival. In North America (Grayson & Meltzer, 2002, 2003) and in Australia (Wroe et al., 2006; Wroe & Field, 2006), this percentage seems to have been small, suggesting that the megafaunal extinction was a protracted process, beginning much earlier than the human settlement of these continents. Such a decline may have been the case in South America, as only a few megafaunal genera apparently survived until the Holocene. While a human presence could have accelerated the process of extinction of the remaining megafaunal genera, climatic fluctuations could also have been responsible. Araujo et al. (2005) suggested a period of drought during the mid-Holocene in central Brazil, based on a general abandonment of the region by humans and also on palaeoenvironmental data. At least for central Brazil, megafaunal extinction could thus be also explained by the dry period that started between 8500 and 7500 14C yr bp (c. 9520–8190 cal. bp). Furthermore, according to Araujo et al. (2005) several authors recognize the existence of dry climatic periods during the early and mid-Holocene in South America. Bush et al. (2005) also found evidence suggesting the existence of this drier period in the Andes region (between 0° and 24°), although in this case it was not a single or synchronous event. Even if asynchronous, the important point here is that this dry period seems to have been a widespread phenomenon in South America. Thus, we concur with Borrero et al. (1998, p. 197) who propose that ‘people played at most a secondary role in the mega mammal extinctions, perhaps accelerating a process already underway before human arrival in South America’. We would like to thanks Rodolfo Salas for his kindness in assisting us in determining the Holocene date in Peru. Our long-term research in Lagoa Santa is funded by FAPESP (grant 04/01321-6) and by scholarships given to AH (FAPESP 04/11485-6), MH (FAPESP 04/01253-0) and to WAN (CNPQ 305918/85-0). Alex Hubbe is a graduate student at the Laboratory for Human Evolutionary Studies, Instituto de Biociências, Universidade de São Paulo. His main interests are the palaeoecology and extinction of the South America megafauna. Mark Hubbe is an investigator at the Instituto de Investigaciones Arqueológicas y Museo, Universidad Católica del Norte, Chile. His main research interest is the origin and dispersion of the First Americans. Walter Neves is the coordinator of the Laboratory for Human Evolutionary Studies, Instituto de Biociências, Universidade de São Paulo. His main research interest is the origins and adaptations of the First Americans. Editor: Mark Bush