Introduction
The extant record on contemporary human biology indicate that relative to other primates, the life history hallmarks of modern humans diverge in many advantageous ways. For example, researchers attribute our greater life history patterns to selective features within our genus, including relatively large brains and advanced dispersal capabilities (Bogin 1999; Flinn 2010; Hill and Hurtado 1996; Kaplan et al. 2000; Leigh 2001). Over the last decade, a mounting body of scientific research has simultaneously called into question and advanced the prevailing scientific consensus on the origin and development of the genus Homo, generally, and the foundations for the identifying features of modern humans, particularly. As an illustrative example, present-day fossil and archaeological discoveries have complicated mainstream interpretations of the early Homo in two fundamental ways. First, recent environmental data from the early history of the Homo indicate that the identifying features of the modern human did not originate from the Homo erectus, from which an “integrated package” of adaptive-humanlike-traits emerged, but, rather a protracted evolutionary time period that encompassed a variation of species. From this perspective, new fossil evidence has expanded the known taxa of the early Homo. Secondly, new lines of interpretation move beyond prior inferences on the context for the evolution of the early Homo and subsequent dispersal from Africa. Overall, the new interpretive frameworks offer new insights on three critical, evolutionary phases in human biology: 1) the arrival of the early Homo, 2) the transition from the early Homo to the H. erectus, and 3) the emergence of selective features (defined in terms of advantageous anatomical differences and longer life expectancies) that aided radical evolution in the genus and the subsequent dispersal from Africa.
Most notably, recent environmental indicators, coupled with a growing body of comparative studies that run the gamut from examinations of mammalian development and energetics to links between developmental plasticity and dietary flexibility, indicate that evolutionary changes in the Homo coincided with climatic variability superimposed on increasing African aridity. Ultimately, the extant record puts the key strategies that gave rise to the successful expansion of our genus into three broad categories: 1) dietary flexibility against the backdrop of climatic variability; 2) cooperative behavior; and 3) reduced extrinsic mortality rates. By dietary flexibility, researchers point to the addition of meet diets of the H. erectus. With respect to the cooperative behavior that emerged within the evolution of our genus, researchers identify two distinct facets of cooperation: 1) teamwork in hunting and the division of labor; and 2) cooperative breeding. Cooperative foraging allowed the later Homo to develop a reliable and flexible diet, suggesting a higher quality diet than the earliest Homo. Cooperative reproduction enabled comprehensive care for offspring. Of course, there remains a paucity of reliable data on the context for these behavior shifts, including specificity on when these traits emerged, and what role, if any, modern life history patterns facilitated our early evolution. Decreased risk of premature death coincided with the ability to reproduce healthier offspring that experience greater life expectancies than earlier species.
Revisiting and Updating Modern Understandings of the Evolution of Homo
Toward the end of the 20th century, a consensus formed that the evolution of the early Homo synced with the progressive expansion of arid conditions and open habitats in Africa, together with the emergence of the H. erectus. From this perspective, the H. erectus ushered in an evolution of key humanlike features that run the gamut, from enlarged brains and long-range mobility to diet flexibility and lower mortality rates – as well as some of the behavioral aspects attributed to the “human package” (Antón, 2003; Shipman & Walker, 1989). These features were interpreted as a foreshadowing of the more recent Homo sapiens, generally, and the emergence of selective features that allowed modern humans to bypass the many of the binding constraints imposed on other species, particularly. For example, unlike other primates, modern humans, beginning with the H. erectus, have hefty energy-expensive brains, high fertility, toolmaking abilities, and prolonged post-reproductive life spans (Bogin 1999; Flinn 2010; Hill and Hurtado 1996; Kaplan et al. 2000; Leigh 2001). This amalgamation of distinct life history traits within the evolved genus tracked the capacity of later Homo, including modern humans to generate higher quality offspring, and thus, eliminating many of the life history trade-offs experienced in other primates (Kuzawa and Bragg 2012). For this reason, the emergence of the H. erectus was interpreted as a radical transition from the early, non-erectus Homo and Australopithecus. As an illustrative example, whereas reconstructions of the H. erectus are analogous to modern Humans, modeling of the early Homo and Australopithecus resemble bipedal apes. Not surprisingly, these morphological differences prompted some to propose new restrictions on the known-rage of early homo, including removing the H. habilis from the Homo taxon (Collard and Wood, 2007).
Present-day discoveries, together with new lines of interpretation (e.g. environmental and archaeological data) call into question long-held beliefs about the origin and evolutionary history of the early Homo. For instance, new fossil evidence has broadened the known range of the H. erectus to include previously excluded small-bodied primates, suggesting limitations in previous reconstructions of the H. erectus (Gabunia et al. 2000; Potts et al. 2004; Simpson et al. 2008;). Some researchers hypothesize these limitations are reducible to two factors: 1) an excessive dependence on the Nariokotome skeleton (KNM-WT-15000) in modeling the H. erectus; and 2) inaccurate analyses of the Nariokotome (Dean and Smith 2009; Dean et al. 2001; Graves et al. 2010; Thompson and Nelson 2011). Additionally, new finds and reinterpretations of previous archeological data point to greater regional morphological variations of the earliest Homo, and highlight underestimated variations and similarities within the H. erectus species (Simpson et al. 2008; Spoor et al. 2007). Additionally, a new line of interpretation on the Australopithecus points to previously overlooked anatomical similarities (e.g. a bulky body and elongated legs) with the Homo. These findings are possibly reducible to previous reconstructions modeled from the “Lucy” skeleton.
A Fresh Perspective
The Origin of the Early Homo
Admittedly, there still remains a paucity of reliable fossil and archaeological data on the earliest Homo. Even so, recent fossil evidence has allowed researchers to formulate reasonable inferences about the non-erectus early Homo. For example, compared to the Australopithecus, the earliest Homo featured above-average brain and body sizes, as well as slower development patterns (Antón 2012; Holliday 2012; Schwartz 2012). If true, a mounting body of research suggests these differences arose most likely in the context of an abundance of food resources, nutritious diets, and decreased risk of premature death, as opposed to emergence of African aridity.
An important observation related to the emergence of the early Homo is the effect of climatic variability on evolutionary biology. The extant geological record links increasingly erratic climatic conditions with the origin of the early Homo, which, according to comparative biology, indicates that the early Homo featured a more pronounced developmental plasticity than the Australopithecus. This hints at the possibility that climatic variability sparked evolutionary adjustments to environmental changing, setting the stage for transformations in dispersal capabilities.
Rethinking the Transformations from Non-erectus Early Homo to H. erectus
Clearly, new archeological data, coupled with new interpretive frameworks for preexisting knowledge, call in to question the inference that the emergence of the H. erectus coincided with radical evolutions in the biology and behavior of the non-erectus Homo. Nonetheless, the extant record indicate that this species differed from other hominins in several key areas. For instance, relative to the Australopithecus, Paranthropus and, potentially, the non-erectus early Homo, the H. erectus experienced a longer life expectancy. Compared to the life history patterns of modern humans and later Homo, including Neanderthals, for example, H. erectus entered adulthood at an earlier age and manifested a shorter adolescent growth spurt (Dean and Smith 2009; Graves et al. 2010; Guatelli-Steinberg 2009; Thompson and Nelson 2011).
Long-range mobility, Climatic Variability, Adaptability and Dispersal in H. erectus
The emergence of H. erectus coincided with a pronounced and widespread long-distance mobility and dispersal across an eclectic milieu of climatic contexts (e.g., the Republic of Georgia and tropical southeast Asia; Antón and Swisher 2004). The archaeological record of dispersal indicates that the population divergence of H. erectus would not have been possible but for the evolution of several key traits common across all widely dispersed living mammals. The most significant of these are: developmental plasticity (Walker et al. 2006); climatic adaptability; a well-developed system of cooperative breeding (Ellison 2008; Kramer 2010; Wells 2012); greater nutritional access; and sociability (Potts 2012; Smith et al. 2012; Swedell and Plummer 2012). Put another way, the extant archeological record indicates three primary reasons that the H. erectus was able to colonize new habitats: 1) a different body composition relative to other hominins; 2) developmental plasticity analogous to modern humans; and 3) greater behavioral plasticity relative to less versatile hominins (Smith et al. 2012).
Conclusion
Recent finds and new lines of interpretation call into question several near-universal assumptions about the selective features attributed to the emergence of the genus Homo and the evolution of the H. erectus as an integrated package of humanlike anatomical and behavioral characteristics. For example, it now seems that many of the selective features attributed to the emergence of the H. erectus arose at different times, and existed in species that predate the H. erectus and fully developed in later Homo. These findings evidence several unforeseen groups that correspond with the anatomical and behavioral characteristics seen in traditional Homo. Additionally, emerging evidence from Africa and beyond establish more pronounced intraspecific variation within the early Homo and expand the range of known H. erectus. The expanded regional morphological variations include both small and large bodied primates. These empirical findings have ushered in fresh perspectives on the morphological variations of the earliest Homo and pacing of evolutionary transformations within the species. In fact, these refined perspectives indicate that future advances will be predicated on how well researchers are able to disentangle the integrated “human package” of traits. In doing so, researchers will be in a greater position to more fully understand and identify the adaptive traits originating in the earliest Homo. This will enable more accurate modeling for purposes of reconstructing the morphological variations within the genus Homo. Perhaps new comparative mammalian studies can fill in the gaps of understanding in the absence of new fossil data. Additionally, a refined understanding on the origin of Homo should aid in the continual goal of anticipating adaptive characteristics across species. Ultimately, further analysis would benefit from a closer look at how adaptability evolved in our genus, as well as the suite of selective features that influenced the developmental and behavioral plasticity of later Homo.
Finally, four continuing goals should guide future research. For purposes of ascertaining the full range of taxonomic diversity of the early Homo, researchers will need to uncover a more expansive collection of fossil samples. Second, researchers should endeavor to create more accurate modeling of the regional morphological variations within the genus. Third, more should be done to further understand the evolutions in the cognitive and dispersal capabilities of the early Homo. Lastly, new lines of interpretation should establish robust modeling of the correlates between life history patterns, energetics, extrinsic mortality, brain and body development, and diet.
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