First of all we applaud Suddendorf & Corballis (S&C) in their efforts to write a much needed, comprehensive review on mental time travel (MTT). We agree with S&C that MTT is a complex, multi-component skill with several advanced cognitive mechanisms working in concert. However, we do not reach the same conclusion when considering the evidence for MTT in nonhumans. S&C argue that the lack of evidence for MTT in animals makes it reasonable that it is unique to humans. We think that one should be agnostic about the human uniqueness claim. The study of mental time travel in nonhumans is so far extremely limited, and actually indicates mostly positive results (e.g., Dally et al. Reference Dally, Emery and Clayton2006b; Mulcahy & Call Reference Mulcahy and Call2006; Naqshbandi & Roberts Reference Naqshbandi and Roberts2006; Raby et al. Reference Raby, Alexis, Dickinson and Clayton2007). Undoubtedly MTT plays a decisive part in human life and society, at the same time as it is extremely hard to observe in nonhumans. MTT is certainly typical of humans, but in what way? It may be that humans are the most apt mental time travelers when it comes to the technical or physical aspects of the world, but that we might not be lonely travelers in the social domain.
In section 6 of the target article, S&C focus on social factors when discussing evolutionary pressures. A problem with this proposal is that it does not explain why advanced MTT should not exist among other apes or animals. Social interaction within many species is very strong. So, if sociality is the crucial factor behind the evolution of mental time travel, it should be expected in some other primates. In fact, we would not be surprised if some primates are proficient mental time travelers within the social domain. The problem is a methodological one: It would be very hard to isolate behaviors connected to MTT when observing the social life of nonverbal creatures; such behaviors could be explained in a leaner way than assuming MTT. If we were to observe humans, or hominins, without attention to language, we would rely on the behaviors surrounding artifacts to identify MTT. De Waal (Reference de Waal1982) noted that the advanced social cognition apparent in great apes probably was a stepping-stone to the complex artifactual cognition in humans. He observes that humans and apes seem almost equally developed in social cognition, though differing widely in technical understanding.
The hand-to-mouth lifestyle of great apes does not apply to humans. In order to understand the typicality and the evolution of human MTT, one must also pay attention to the differences in ecology. Following Osvath and Gärdenfors (Reference Osvath and Gärdenfors2005), we argue that the Oldowan culture, 2.6 to 1.5 million years ago, constituted an ecological niche that enhanced the ability to mentally represent a possible future. MTT might to some extent already have been in place as a result of earlier selection pressures in the social life, but changes in hominin ecology chiseled out the more advanced ability that we seem totally dependent upon nowadays. The main components of the Oldowan culture are recognized as: (1) the manufacturing and use of stone tools; (2) the transport of artifacts (at least stone tools); (3) the transport of pieces of carcasses; and (4) the use of accumulation spots (Plummer Reference Plummer2004). A significant advantage of this culture is that it enabled a much wider exploitation of meat resources.
Savannah conditions offered some hominins a wider variety of food sources, and these food sources were more transient and scattered than those exploited by other primates. Therefore, the day ranges of the early hominids were more extended than those of extant apes (cf. Bickerton Reference Bickerton and Wray2002). The Oldowan life style was signified by an extension in time and space. The fitness of the hominids in this niche increased with adaptations for long ranging, as indicated by the skeletal remains. These morphological adaptations must have been related to behavioral adaptations, which could be a result of an evolving mental time travel.
One example, supporting this general argument, concerns the curated technology of the Oldowan culture. As S&C mention, there is clear evidence that transport of the artifacts (at least the stone tools) was an important trait of the culture (Toth Reference Toth1985). It is not possible to know exactly where the next fresh kill will be found; it might be several kilometers away from nearest raw material source. Without sharp-edged stone tools in the immediate vicinity, a carcass would lose much of its value. Another aspect of Oldowan culture seems to be the saving of a tool (or a core) after it has been used.
A second example concerns the division of labor. This factor could in fact be used to turn the group of hominids into a virtual Swiss army knife. Some individuals might carry throwing stones, some might carry children, and others could carry water or wooden tools; some may go scavenging or hunting while others may focus on gathering food. Division of labor in the modern human form of hunting and gathering is deeply dependent on mental time travel. The individual must in some sense be able to imagine other individuals currently outside his or her immediate sensory scope doing their part of the job.
These examples present some reasons why complex mental time travel evolved to the human level within the Oldowan culture – or earlier (for a more detailed account, see Osvath & Gärdenfors Reference Osvath and Gärdenfors2005). Once the period of Acheulean tools was reached, beginning about 1.5 million years ago, mental time travel was in an advanced stage.
MTT might not be unique to us, but we have the highest physical-technical accuracy and focus in our travels. And, we are probably also the species most dependent on MTT – the world in which we can survive is a world manipulated and constructed by complex artifacts, so our mind must be highly tuned to these aspects. This might be the prime reason for why we constantly observe MTT in Homo sapiens.
