Patterns of Growth and Development in the Genus Homo (Cambridge Studies in Biological and Evolutiona

Cambridge Studies in Biological and Evolutionary Anthropology Patterns of Growth and Development in the Genus Homo. It is generally accepted that the.
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Recent approaches have neglected to examine stature in favour of body mass, with no study analysing the subtle interactions of both size components. The scope of these studies, however, did not allow the contextualization of variability in body size throughout the Plio-Pleistocene.

Several methodical problems and limitations impede the study of body size from a long-term and inter-taxonomic perspective. While these studies are tailored to their specific research question and provide individual estimates of high accuracy, such an approach has led to a neglect of the amount and importance of variability, with the extent of intra- and interspecies variation in body size remaining largely unknown. The prevalence of small sample sizes also more generally affects the study of long-term and inter-taxonomic patterns in hominin body size evolution and the application of statistical methods.

Even when estimates for individual specimens are accurate, means of such groups with small sample sizes are unlikely to adequately estimate original population means and might thus not be representative for past demes. The low signal-to-noise ratio for such samples can only be enhanced by increasing sample sizes. According to the principle of statistical regularity, an umbrella term that also encompasses the law of large numbers, statistical regularities emerge when sample size increases, meaning that random and rare variation or error has less weight on overall patterns.

Using large sample sizes also aids in finding patterns that are not detectable using restricted samples. In sum, small sample sizes constitute the key limitation for large-scale studies across the whole timeframe of hominin evolution which necessitate as large samples as possible to increase the signal-to-noise ratio and examine patterns of body size evolution in appropriate detail at various levels.

In order to assess long-term and inter-taxonomic patterns of body mass and stature evolution within the hominin lineage, the general approach of our study was to tackle the essential problem of small sample sizes for hominin body size estimates by gathering as many reliable predictions as possible from our own work and published sources.

Importantly, the large sample allows the application of statistical methods to test for inter-group differences. We characterize change in both size and variability through human evolution and assess more specific hypotheses divided into temporal 1 and taxonomic 2 questions: Did body mass and stature evolve in concert or follow separate trajectories?

Does variability in size change through time? We combined our own estimates of body mass and stature among early Homo between 2. For body mass estimates greater than 2. The period after 1. Selection criteria for estimates in the Sima de los Huesos palaeo-population are provided in electronic supplementary material, text S1 and file S2. In total, our sample encompasses size estimates of hominin fossils of several species and genera Homo: The fossil hominin dataset covers roughly 4.

Our approach of maximizing sample sizes of hominin body size estimates to detect large-scale patterns with statistical methods comes with the cost of incorporating individual body size estimates from different studies, estimation methods and skeletal elements which introduce errors. These methodical trade-offs are common in meta-studies and can be justified by the rationale and aims of the study see above.

While a complete removal of error sources is impossible, control of quality and commensurability of body size estimates was an important concern of the study design in order to minimize these problems as far as possible. Methods for these controls are described in the following in order to provide transparency and replicability of our approach.

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Results of this approach were checked for consistency against results deriving from the entire database, ensuring that they do not bias the overall analysis see summary in electronic supplementary material, text S2. This approach was necessary due to large differences in body mass estimates for some early Homo fossils greater than 1.

However, we decided to include them—trading data quality for quantity to a certain extent—in order to: These trade-offs can be justified by the law of statistical regularity, whereby increasing sample sizes will also lead to a better differentiation between signal and noise in the data. Importantly, another advantage of this approach is that it allows for the application of statistical tests which most previous studies of predominantly small sample sizes were not able to do.

Our analysis of estimates follows along two independent variables: Chronometric ages were taken from the literature. We focus on temporal analyses as time is a more reliable variable than taxonomy. Body size was regressed on absolute time to assess general patterns and timing of change. The groups are provided in electronic supplementary material, table S5, with the attribution of individual specimens to these groups found in electronic supplementary material, file S1. Means and coefficients of variation were compared between groups to identify potential periods of step increases or gradual change.

This approach, however, introduces increased error in mean values for any given time period i. In order to gain a better understanding of evolutionary rates within evolving lineages, a confident hominin phylogeny with clear ancestor—descendant relationships is required; however, there is currently no consensus.

The application of a hypothetical phylogeny to the analyses thus risks the introduction of additional errors. We took into account taxonomic designation of the hominin fossils, both to the level of the genus and the species where possible. While a broad assignment to the genus level is considered to be more robust, the alpha taxonomy of some specimens—particularly for early Homo and generally hominin fossils between 2.

The groups are provided in electronic supplementary material, table S6, with the attribution of individual specimens to these groups found in electronic supplementary material, file S1. The taxonomic information for these groupings was taken from the literature with the principle of majority rule applied. To assess variation in body size in relationship to other independent variables, we performed statistical analyses on the Plio-Pleistocene hominin sample using SPSS Mean and median values were calculated for all groups based on reported point estimates for individual specimens. Post hoc comparisons among more than two groups were made using Games—Howell or Mann—Whitney U -tests with Bonferroni's correction to protect against Type I errors.

Nonlinear line-fitting provides slightly higher R 2 -statistics but comparable or lower F - and p -values electronic supplementary material, table S7. While these different models indicate a chronological trend of increasing body size through the past 4. The temporal pattern of generally increasing size is interrupted at two points: Body size estimates by time in the entire sample of fossil hominins.

Comparing hominin samples partitioned by coarse temporal groups electronic supplementary material, table S5 shows that both Middle and Late Pleistocene hominins feature large body masses on average approx. Results are nearly identical for stature: Body size estimates by coarse temporal groups. Results of the chronological analyses of hominin body size estimates by coarse temporal groups. The lowest body mass values are found in the Late Pliocene and early Early Pleistocene mean: The largest and youngest group encompasses the middle Middle Pleistocene as well as the Late Pleistocene mean: Testing these findings by different methods electronic supplementary material, text S3 and table S2 underscores the robustness of the marked and significant increases.

An important difference from the analysis of body mass is an earlier shift in stature between the middle and late Early Pleistocene approx. Body size estimates by fine temporal groups. Results of the chronological analyses of hominin body size estimates by fine temporal groups. Reduced levels of variation without overlap to these temporal categories characterize the late Early Pleistocene to Late Pleistocene range of CVs: CVs of both temporal analyses suggest a relatively consistent decrease in body size variability through time—except for body mass in the late Middle Pleistocene with the inclusion of small-bodied Homo naledi see below —with the most recent time slices exhibiting CVs comparable to or slightly below the Holocene sample.

An early group with the lowest body mass including Ardipithecus , Australopithecus and Paranthropus mean: Differences between early Homo and Homo naledi are not significantly different in this analysis. Body size estimates by broad taxonomic groupings. The Holocene forager sample is displayed separately. Taxonomic analyses of stature find a first marked increase in early Homo compared with Australopithecus and Paranthropus approx. The second larger increase in mean stature is temporally decoupled from body mass, taking place earlier between early Homo and early Mid-Pleistocene Homo approx.

A separate analysis assessed body size around the origin of Homo. The early Homo sample for this analysis includes all specimens between 2. This approach excludes large-bodied forms of Homo after 1. These findings are robust to different estimation methods electronic supplementary material, text S3; table S3 and exclusion of potentially aberrant specimens electronic supplementary material, text S2; table S1. Body mass by time with indication of taxonomic group membership. Stature and ponderal index by time for fossil hominins and Holocene foragers.

Higher values indicate relatively stockier builds. Body size estimates by narrow taxonomic groupings. Taxonomic analyses of hominin body size estimates by fine groupings, ordered through time from younger to older. Temporal analyses within taxonomic categories test for diachronic change in body size among these groups.

The majority of taxonomic groups exhibit no linear change. Regression slopes indicate a gradual increase of body mass throughout time in Neanderthals and early Mid-Pleistocene Homo , whereas australopithecines, early Homo and UP modern humans show a decline. All other groups do not exhibit significant associations of stature with time, suggesting within-group stasis. Within-group increase in early Mid-Pleistocene Homo is driven by a rapid shift towards larger body mass after 0. This marked change is found in non- Homo erectus specimens e. These observations support a step increase in body mass within Mid-Pleistocene Homo after 0.

In sum, most taxonomic groups exhibit internal stasis with some taxa demonstrating gradual change over time. There is, however, a high degree of variability for each analysed temporal group of hominins. Starting from a purely descriptive perspective, the pattern of increasing body size through time is interrupted by marked reduction in stature and body mass among australopithecines between 3.

Within the age bracket of 3. While Middle and Late Pleistocene Homo are indistinguishable in their large body mass from one another, they exhibit significantly higher values compared with Early Pleistocene and Late Pliocene hominins. At the end of the Late Pleistocene, the analyses found a gradual within-group decrease of body size for UP modern humans.

Our temporal analyses demonstrate that increases in body size throughout the past 4.

While chronological trends across lineages cannot be equated with evolutionary rates, the resulting pattern of relatively rapid increases within short timeframes is supported by within-group analyses that show stasis for many taxonomic units. Micro-evolutionary mechanisms include directional anagenetic change within lineages i. Our analyses suggest that such directional changes occur only within few taxonomic groups, whereas marked differences are often found between temporal and taxonomic units.

Any of these macro-evolutionary processes could produce the punctuated net increases in size between the temporal groups found here. While discriminating between such interpretations requires testing with formal hominin phylogenies, the following combined discussion of temporal and taxonomic results can provide some initial insight into the evolutionary processes behind these patterns. The first step change coincides with the earliest fossils of Homo in our sample—as well as a decreasing number and last appearance datum of several australopithecines—and might thus result from a mixture of cladogenesis and the extinction of smaller-bodied forms.

This period of change is followed by a major shift in stature already in the late Early Pleistocene, long before respective increases in body mass. The second major increase in body mass is found only at ca 0. Summary of mean values and variation in hominin body size estimates. Double line graphs for: New postcranial fossils from currently underrepresented time periods—such as 3. These issues are most evident for the small-bodied Homo naledi specimens dating to ca 0.

Only further phylogenetic analyses will be able to answer whether the small body size of this species is due to a retention of ancestral small body size, hinting at a long and hitherto unrecognized persistence of archaic traits in postcranial build, or rather a convergent trait that evolved anew in this lineage due to localized selection pressures. The existence of even smaller-bodied individuals of Homo floresiensis in the Late Pleistocene raises similar questions and shows that this is not an isolated case.

This study also found marked differences in absolute and relative variability in body size throughout time that correspond with broad taxonomic units. The incomplete nature of the fossil record also implies that some extinct populations have not been sampled and additional biases e. Our method may have also increased the amount of variation observed compared with studies which use a consistent estimation method on a single skeletal element only.

This being said, testing the effect of the use of different methods and number of studies involved per analytical unit did not change the observed relative patterns of variation to a large extent see electronic supplementary material, text S3; tables S2 and S3. These findings demonstrate abundant change in variability, but no consistent linear pattern of increasing or decreasing variation throughout time.

The entire variability spectrum of present humans is only attained with the Holocene foragers. We conclude that there have also been two major shifts in body size variability, which do not easily correlate with transitions in mean size. The first shift takes place between the Early and Middle Pleistocene—within the earlier part of the genus Homo —during which a selection against smaller body mass and stature occurred in most taxa i. This process narrowed the range of body sizes by shifting the overall spectrum towards larger bodies, with Homo naledi and Homo floresiensis as notable deviations from the general pattern.

The high variability in body sizes during the middle Early Pleistocene 2. These observations highlight the importance of studying intra-taxon variability and point to a potentially elevated role of phenotypic plasticity in the evolution of early Homo , as well as Homo erectus s. Combined temporal and taxonomic analyses of changes in body size have the potential to shed new light on other debated issues in human evolution.

The various tests for both body mass and stature of this study are consistent with previous results in that the origins of Homo are characterized by a significant increase in body size compared with Australopithecus and Paranthropus. Early Homo specimens between 2. The difference between early Homo specimens and Au. Although the oldest representatives of Homo in our analyses are significantly larger in body size compared with contemporaneous and immediately preceding australopithecines, the lack of postcranial fossils and body sizes estimates for earliest Homo between 2.

At the same time, fossils of early Homo between 2. Our taxonomic analyses of an enlarged sample identified similar patterns, but with two major shifts in different body size parameters. A first marked increase in stature—and a minor one in body mass—took place between early Homo including Homo habilis and Homo erectus s.

This decoupling in body size parameters corresponds with the temporal results, suggesting an earlier increase of stature in Homo erectus s. The ponderal index shows a slight decrease throughout time, but with taxonomic differences among later Homo. Early Mid-Pleistocene Homo e. Homo erectus and Pleistocene Homo sapiens show consistently low values, with the minimum reached by predominantly African MP Homo sapiens. Later Homo species thus retained the tall stature from Homo erectus s. Late Pliocene and Early Pleistocene hominins are characterized by more variable body forms i.

Our study adds new perspectives to hominin body size evolution with its focus on long-term and inter-taxonomic patterns and variability through time in both body mass and stature. Long phases of stasis indicate only minor anagenetic increases within many taxonomic groups analysed here e. Homo erectus ; Australopithecus africanus. The marked and seemingly rapid shifts in size through time, particularly in the earlier Early Pleistocene, could be the result of cladogenesis i.

The findings suggest selection against small body size operating from ca 1. While the reported trends in later Homo apply to most recognized taxa, the appearance of small-bodied individuals in the late Middle Pleistocene Homo naledi and Late Pleistocene Homo floresiensis suggests additional layers of complexity to the evolution of body size in the genus Homo. Beside issues of causality, other open questions and challenges remain. The scarcity of postcranial fossils for crucial periods—for Homo between 2.

While one of our main goals was to increase sample size as much as possible in this study—with the trade-off that this approach necessarily includes estimates based on different studies, methods and elements that are not all of the same accuracy—future discoveries and larger-scale analyses will be required to more broadly test our results. Nevertheless, methodical advancements will be necessary e. Our results have important ramifications for studies concerned with human energetics, dispersal and encephalization, but also more generally for how we interpret the evolution and biology of our genus.

Mosaic evolution and the pattern of transitions in the hominin lineage

In particular, this study underscores the large variability in body size in the hominin lineage and the complex pattern of its evolution throughout time and among taxa. Rather than focusing exclusively on species means and unidirectional models, perspectives that emphasize variation and nonlinear patterns within multidirectional trajectories might thus be fruitful strategies for interpreting the evolution of body size in our lineage. We acknowledge support by the many people from different institutions that gave access to the important human and fossil collections under their care, and kindly provided assistance.

We thank the reviewers and associate editor for their valuable contributions which have helped to improve this article. All of our raw data and materials are available as electronic supplementary material electronic supplementary material, file S1 or are part of this paper. Broadly speaking, these can be considered to be in the Pliocene, during the Plio-Pleistocene and in the later Quaternary. It should be noted, however, that these represent very different scales—the first two covering more than a million years, the last less than half a million years.

The resolution with which we can see changes is thus very different, and to refer to them as if they represent the same mode and tempo is probably misleading. The third observation is that each of the three periods of transition is distinctive in its character, relating to different aspects of hominin and human adaptation. The Pliocene transition, in as much as the evidence can show it, appears to be related to patterns of locomotion and ranging behaviour, suggesting a novel habitat and ecological niche, arguably as the environment became more dominated by woodland and grassland.

Inevitably, there would have been shifts in diet, behaviour and socioecology as the populations responded to the new environments, but the absence of archaeological evidence makes this hard to detect. The evidence suggests that the degree of committed terrestrial and arid specialization and adaptation was unique among apes. In other aspects—cultural transmission and cognition, for example—it is likely that the adaptive zone of the earliest hominins would have been not substantially different in scale from that among other ape species.

The Plio-Pleistocene transitions are complex, and far better documented. These would be said to occur across the period from about 3. The earliest elements of this transition would be the appearance of stone tools at Lomekwi dated to 3. The early part of this transition 2. This becomes more unified after 2. The evidence for technology for the early part of the period is very limited, but from about 1. At about the same time, evidence for butchery of animals, possibly as a result of hunting, increases markedly [ 79 ].

The end of this period is also associated with the extinction of the australopithecines, the evolution of transitional and early members of the genus Homo , and the paranthropines, suggesting a substantial shift in niche structure, and overall a new adaptive zone for hominins.

It also appears to be the basis for the first dispersals into northern Africa and Eurasia [ 80 , 81 ]. However, perhaps the major point to emphasize for this complex behavioural and life-history transition is that it is not a single compressed event, but spread over more than a million years, and likely to be the product of multiple smaller microevolutionary shifts. That this is not entirely the fully novel adaptive zone of humans can be seen by the extent of change that occurs one million years later.

This could be summed up as the evolution of H. This Late Quaternary transition is centred on major behavioural, cognitive and cultural changes [ 83 , 84 ] and references therein. There is a substantial increase in brain size across the period, and changes in cranial morphology and overall robusticity, but compared with the physical changes taking place in the earlier transitions, these are relatively minor.

However, in behavioural and cultural aspects there is a major change, both in the development of new traits, and also in the rate of change. The key elements of this phase of human evolution have been well-rehearsed—a ratcheting of rates of change and increased complexity in technology [ 85 ], the emergence of regional entities and identities [ 86 ], greater population densities [ 87 ], evidence for enhanced cultural processes [ 88 ], symbolic thought and representation [ 89 ]. The rate is significant too.

The period of time involved, less than 0. The multiple events of the evolution of modern humans. The evolution of modern humans is a very rapid event in the context of evolution as a whole, but is nonetheless composed of many dispersed events or transitions. Each of these and others not yet discovered contributed to the totality of the modern human transformation. Admixture events between populations not shown.

There is little doubt that humans occupy a novel adaptive zone, unexplored before. In this context, it can be safely argued that human evolution comprises to a large extent the third level of evolutionary change, comparable with the first land creatures. However, the wealth of archaeological and fossil evidence indicates strongly that the change occurs across the whole of the seven or less million years since the divergence from the last common ancestor with chimpanzees, and actually consists of three separate phases of substantial adaptive change.

The first of these is related to locomotion, foraging and habitat adaptations; the second to a suite of behavioural changes that are linked to a change in diet, means of acquisition of resources technology and life-history strategy; and the final one is strongly based on cognitive and behavioural changes.

The key criteria are the emergence of larger entities of replication, a division of roles, the loss of independent replication, resulting in evolutionary fragility. The transition results in novel ways of transmitting information. There are several obvious candidates that could lead to such a transformation—technological dependence, language, cumulative culture, high levels of reproductive cooperation and cooperation beyond kin-related individuals. To some extent they are all inter-related, such that it is probably impossible to untangle which is the key element.

However, it is likely that the underlying extreme levels of social cooperation, both for breeding and for constructing social tolerance, are as much at the centre of the process as language itself.

Equally, it is unlikely that the high levels of communication and cooperation, which form the basis for modern society, would be possible without technological abilities. Furthermore, the evidence we have explored at a lower level of evolutionary transition shows that the evolution of humans is not a single event, but a process of combination and accumulation. It is not one phase of becoming human that represents a major transition, but the cumulative effect of them, the processes of mosaic evolution, and the very recent extinction of all other hominins that enhances the distinctiveness of humans.

Only the extraordinarily detailed resolution of the recent fossil and archaeological records provides that insight into major evolutionary change. While there may be some doubt about human evolution as a genuine radical transformation in evolution, there can be none about its consequences. In terms of rates of environmental change caused by humans, the impact on rates of extinction, and the consequences for life on the Earth, there can be no doubt.

It has also been argued that human impact in the Holocene has resulted in the first major restructuring of trophic systems since the establishment of terrestrial herbivory in the late Permian [ 91 ]. In that context, the evolution of humans is a major and irreversible transition. In posing the question of whether humans represent a major evolutionary transition, it was never the intention to provide a categorical answer.

Such terms are analytical concepts, not biologically meaningful units. However, in asking the question, we can explore the processes by which humans did develop a unique and un-controversially different evolutionary profile. First, if unsurprisingly, that human evolution is a gradual and cumulative process, best described as mosaic evolution [ 92 ]. It is worth considering briefly what is meant by mosaic evolution. At the most local level it simply means that within a lineage, different traits evolve independently and at different times; this is the basis of Hublin's accretion model of Neanderthal evolution [ 93 ].

It is likely that within any lineage mosaic evolution at this level will occur, although due to pleiotropic effects there may also be degrees of coevolution, producing a more correlated evolutionary pattern. Thus, different traits appear and change at different times, and the rates of evolution vary not just between periods but also between elements of the hominin phenotype and extended phenotype. At a higher level, though, mosaic evolution is when different domains of evolution change at different times.

Thus, one part of a lineage's history might see rapid changes in dental patterns, while during another phase it is body size that changes. The pattern of hominin evolution described here fits this higher level form of mosaic evolution. This is not just the case leading to the origin of modern humans the last transition , as it is clear that since the appearance of H. Second, the three transitions identified within a broader pattern of change are different elements of the mosaic; at its broadest level, the first is about the changes in how hominins ranged across the landscape; the second is about the nature of the resources they acquired, and how they acquired them; and the third is about changes in reproduction and sociality.

Only when this last was in place do we observe the full impact of cultural evolution as a rapidly accumulating process. This sequence—ranging, diet breadth and resource extraction, and socioecology—can be seen as the necessary building blocks for being a modern human. What would be interesting is to explore further whether this is a sequence replicated in the evolution of other lineages. Third, following on from this, it can be argued that these building blocks depend upon ecological foundations.

There has been considerable discussion in studies of human evolution about the social brain and social factors driving hominin evolution, but such a view can only hold if a relatively short period of time in the evolution of our lineage is considered. The totality shows a strong ecological foundation. Fourth, it is clear that behaviour—defined broadly, and including the later cultural mechanisms of behavioural innovation and transmission—plays a central role in the process.

Approaches to human evolution have traditionally focused on morphology, as fossils have been the source of information, and more recently genes, as these provide excellent markers of evolutionary history, but in each of the major transitions behavioural changes can be seen not just as important, but also chronologically earlier. This would lead to further incorporation of behavioural processes in models of evolutionary transitions e. Baldwin effect , and in evolutionary theory more generally [ 95 ]. Finally, it is worth stepping back and returning in a different way to the questions posed at the beginning about major transitions.

Whether formally a major transition or not, humans are the product of major changes since the last common ancestor with apes, and this takes place over a period of 5—7 Myr. Parts of that evolutionary sequence can be observed on a millennial scale, and all within a resolution of tens of thousands of years. Had this been an evolutionary event occurring tens or hundreds of millions of years ago, such resolution and visibility would not be possible. Furthermore, the hominin habit of making and discarding stone tools provides a unique record of behaviour. It is that extension of the fossil record and the high level of palaeobiological visibility that allows us to see how major, macroevolutionary transitions are embedded in a sequence of microevolutionary ones.

Human evolution, it turns out, is not just interesting in its own right, but for the insights it provides into evolutionary processes in general. National Center for Biotechnology Information , U. Accepted Apr Published by the Royal Society. This article has been cited by other articles in PMC. Abstract Humans are uniquely unique, in terms of the extreme differences between them and other living organisms, and the impact they are having on the biosphere.

Introduction Evolution—that is evolution simply as change through time—can be broken down into two elements. Evolutionary transitions Evolutionary change can be as small as a minimal change in the number of hairs on a Drosophila , to an entirely new means of reproduction. Open in a separate window. Transitions in human evolution: Evidence for the different levels of transitions in human evolution a Baseline evolution There is ample evidence for simple, baseline evolution across the span of human evolution. Discussion In posing the question of whether humans represent a major evolutionary transition, it was never the intention to provide a categorical answer.

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1. Introduction

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