The mosaic evolution of human species

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The emergence of the Homo sapiens was not limited to a single event. It was the consequence of a long process of crossbreeding and combinations between the proto-sapiens populations, then between Sapiens, evolving in different regions within Africa (and outside it).

„Every being is a multitude.”
W.G. Goethe

So, how did humans emerge? I believe the most accurate way to explain the human genesis, is through the concept or hypothesis of mosaic evolution. This is an integrative model that overarches the existing contradictions and can shed light over the complex origins of our species: its adaptive transformation and connections with other hominins. It’s a theoretical approach, in agreement with most contemporary research and discoveries. In the following pages I shall try to present the core of this concept, hoping that future research will prove its veracity.

It must be said that evolutionary theory is already acquainted with the concept of mosaic evolution, however, according to its standard expression it has a rather limited meaning. In particular, it is related to the fact that during the evolution of any given species, different physiological features and organ systems were developed unevenly, following different rhythms and stages. Simply put, whilst some anatomical structures evolved, other were preserving their original, archaic structure. For example, the bipedal locomotion of the first hominins, as a behavioural and morphologic adaptation, has anticipated (and lead to) a broader pelvis and cranium [McHenry, 1975].

Compared to humans, a mosaic pattern has been identified in the morphological evolution of the muscles’ of chimpanzees (Pan troglodytes) and bonobo (Pan paniscus). Some anatomical features are common for chimps and humans, some are common for bonobo and humans, while others, the majority, are common for the two chimp species [Diogo et al., 2017].

A mosaic structure, with features both archaic and modern, had the hominin skulls, especially the intermediary links of the species. The German anthropologist Gunter Brauer has argued in his studies that the anatomical evolution of the human species in Africa features a mosaic-like process [Brauer, 2008]. Supporting this thesis is the research related to the composite cranial architecture of the skull discovered at Jebel Irhoud [Hublin et al., 2017].

Recently it’s been discovered that during the millions of years of evolution, the height and weight of the hominins progressed at fluctuating rhythms, alternating evolutionary stasis and rapid evolutions. Generally speaking, it is believed that the height has long anticipated the body mass, and it is assumed that this has led to the supple shape of the Homo specimens bodies. These features allowed for a body suited to long runs in the arid savannah environment, whilst the weight gains progressed more rapidly after the migration to colder plains [Will et al., 2017]. A typical presentation of the mosaic evolution stresses that various adaptive categories – morphological, physiological and behavioural – specific to the modern humans did not emerge concomitantly, but were formed independently one from another during the evolution of the hominins. The adaptations of the hominins to the environment were composite and cumulative, as a consequence of a “mosaic of trends” [Foley, 2016].

Beyond these thesis, the notion of mosaic evolution can reach a new conceptual and paradigmatic meaning, which can be broader and deeper. Not only the appearance and emergence of the features and adaptations was mosaic, but the process of human evolution itself (and supposedly, of most species as well) was mosaic-like. It is easy to imagine how such processes occurred. It is generally known that the appearance of families, genres and species is accompanied by a process of continuous diversification, leading to the formation of a phylogenetic tree of populations or taxonomic units, distributed across ecological and geographical niches. Likewise, between more or less kindred units, there can occur breeding and hybridisation, which can lead to new intermediate varieties, which will share mutations and features common to other units of the same genre. Visually, it’s like a puzzle, where each element contains parts belonging to other elements; or, in the same topic, each unit is made of a mosaic of features, characteristic to other units as well. I would like to emphasise that the species have a mosaic composition, formed of different populations, each having various ecological adaptations.

A fine example of the pattern of mosaic evolution can be discerned in the emergence of a recent species of Galapagos finches – following the hybridisation of two different species. It’s amazing that the new taxonomic unit appeared within such a short time span, because the hybrids, resulted through the fusion of the two founding species, were reproductively isolated and practised a very tight inbreeding; the whole process has been observed and described by the famous researchers of Galapagos finches – the Grants. Therefore it was a process of fast hybridisation (within several generations) leading to the appearance of a new species of the same genre [Lamichhaney et al., 2017]. We can admit that, stricto sensu, the evolution of every phylogenetic line is characterised by speciation, “side steps” and produces a group of taxonomic units related, yet distinct, which interact and exchange genetic information. It is like the evolutionary  processes would be characterised by a ceaseless combinatory interaction. In the above mentioned example, it can be perceived that within the mosaic of the Galapagos finches a new unit occurred, and the new taxon is itself mosaic, as it combines genetic elements of both founding species.

In a somehow similar way the Italian sparrows (Passer italiae) appeared. It is the result of a crossbreeding between the house sparrow (Passer domesticus) and Spanish sparrow (Passer hispaniolensis), which occurred around 10 000 years ago. Later on, the hybrid generation was reproductively isolated from the parent populations and nowadays the Italian sparrow has become a separate species. In their genome, the DNA proportion is around 62% house sparrow and 38% Spanish sparrow. The researchers are using the notion “mosaic parental inheritance” to define the parental genetic combination and it’s been noticed that the Italian sparrow species is characterised by a “genetic and phenotypic mosaicism” [Elgvin et al., 2017].

Additional example of mosaic-like evolution is represented by the common raven (Corvus corax). A study of the population of North American ravens has shown that almost 1.5 million years ago they were divided into three genetically distinct lines. Later on, two of these lines merged into a common group, featuring new combined traits (that were dissimilar to the original populations). Hence a living illustration that speciation is not made solely through the divergent evolution of various branches, which, in turn, will lead to the formation of other branches. It is also through the merging of the once separated branches, which will lead to mosaic genome populations. The researchers have called this phenomenon “reversal speciation” and “reticulate evolution”, meaning a web or thread-like evolution, as opposed to the classical “tree and branches” evolutionary model [Kearns et al., 2018]. This evolutionary model implies the repeated interbreeding of species and populations erstwhile estranged and it is surprisingly close to the Sapiens/Neanderthal hybridization.

Another curious example, which reveals a slightly different perspective of the appearance of the species and races, refer to the origin of the domesticated horse. For a long time it’s been considered that modern horses derive from a common ancestor, domesticated nearly 5 500 years ago by the people of the Botai Culture (Middle Asia, Kazakhstan). Recently it’s been demonstrated that, actually, the origins of the horses is far more complex, as only a small part of their genome relates to the Botai line. There are two possible explanations so far. Either part of the Botai domesticated horses, spread across wider regions, crossbred with other species of wild horses, inheriting their genetic legacy, or, there were several areas of horse domestication, and later on mixed breeding occurred [Gaunitz et al., 2018].

Talking about the hypothesis of mosaic evolution of human species, it’s worth to notice some resemblances with another, almost homonymous evolutionary concept – the geographic mosaic theory of coevolution. Briefly, its authors are claiming that species are often collections of genetically distinct populations, in other words, there is a vast intraspecific variability. These populations, living in various geographical areas, will interact in different ways with the environment, as well as with other species of plants and animals. For example, the interactions with some organisms can be antagonistic in some environments and cohabitational in others. Species of insects can be pollinators for species of plants in some areas (mutuality), whilst in other areas same insects can use the nectar or the flowers of those very plants without favouring their reproduction (floral parasitism). In this matter various results were recorded in different environments. It’s being suggested that, although belonging to the same species, populations spread across various geographical niches, can create a mosaic of interactional patterns with their environments. These co-evolutionary processes, subjected to local selective pressure, and accompanied by the acquisition of new mutations, will generate a continuous and divergent modification of populations, where local models will be consolidated without becoming typical for the whole species [Thompson, 1994, 1999, 2005; Gomulkiewicz et al., 2000; Thompson, Cunningham, 2002; Anderson, Johnson, 2008; Piculell et al., 2008; Hoeksema, 2012]. This theory has its own particularities, as it is centred on the co-evolutionary phenomenon; it explores the interaction between different taxa (such as insect versus plant or plant versus fungi). We can observe that even for the diversified co-evolutionary phenomenon, the mosaic concept proved to be efficient and illustrative.

The origins and history of the hominins would be logically placed within a similar evolutionary mosaic-like model. Under the relentless pressure of natural selection, the hominin populations were acquiring continuous adaptations to the new environments in different geographical areas, as well as under the impact of the climatic changes. This is how the divergence and multiplication of hominine populations was stimulated. Some preserved their distinctiveness, whilst others have interacted and merged, generating mixed types. It must be said that the speed of these evolutionary processes fluctuated as well. Concerning the Neanderthals, recent studies confirm that they’ve evolved not only gradually, through small and cumulated transformations but also following the pattern of punctuated equilibrium, when long periods of stasis are being followed by spurts of fast changes, nearly spontaneous when reported to the evolutionary chronology. Likewise, the Neanderthals evolution seems uneven on a geographical level, as in different regions these populations have acquired various adaptive modifications, have hybridised differently with other hominins, at different stages of their history [Zilhão et al., 2017].

Moreover, a theory has been proposed, suggesting that the origin of the Neanderthal is not exclusively European (maybe not European at all). Based on the compared study of the Neanderthals dental morphology, it seems that they had a mixed genealogy, and that there were at least two evolutionary lines, one of them originating, probably, from South-West Asia. Both populations have been subjected to the processes of geographical isolation, interaction and hybridization which means that the Neanderthal evolution wasn’t linear, as previously assumed, but puzzle-like [Bermúdez de Castro et al., 2018]. In other words, it is safe to assume that the Neanderthals had a mosaic evolution.

Through similar patterns of poligeographic and multiple genealogy can be explained the incredibly mixed background of Sapiens diversity from Paleolithic to modern era. It seems that our immediate ancestors, the so-called proto-sapiens, had a wide area of diffusion in Africa (perhaps outside Africa as well), where they’ve been through the mechanism of unequal evolutionary processes and mated with other species, closely enough to create crossbreeding. Due to these factors a continental (maybe a cross continental) scale mosaic appeared. From the beginning there were several lines of populations who could have been the ancestors of the first Sapiens (having local, endemic features), sharing a series of similar traits (such as high cranial vault or prominent chins). There is no such thing like a single area and exact date for the appearance of the human species in its complexity. Due to the fact that various proto-sapiens populations were endowed with just some of the specific traits of the species, only after their crossbreeding, over the tens of millennia, those traits were combined in a whole leading to the emergence of the anatomical profile of the modern Sapiens.

I hereby present an image I deem to be illustrative for the tri-dimensional mosaic pattern for the distribution and interaction of the proto-sapiens populations prior to the actual emergence of the Homo sapiens. Each sphere represents a population that interacts with other populations, slightly different genetically and phenotypically, but they all together lead to the genesis of a united species.

Image Source: http://atoz-desktop-wallpapers.blogspot.com/2013/08/3d-mosaic-spheres.html

As mentioned by the British paleoanthropologist Chris Stringer, when we examine the emergence of the human species in Africa “we need to talk about origins, rather than a single point of origin” [Stringer, 2012, p. 253]. Indeed, in Africa coexisted several archaic types of Sapiens which, supposedly, either lived in a total isolation or interbred sporadically. This thesis is supported, for instance, by the finding of the Iwo Eleru skull, which is very archaic in its morphology (towards 200 000 years old) however, surprisingly, it has been found to be only 13 000 years old (therefore relativelly recent). The scholars consider that this odd incompatibility, between the archaic morphology and relatively recent age, proves that some archaic sapiens populations have coexisted for a long time in parallel with the populations of modern Sapiens [ibid., p. 254-255]. Stringer righteously questions whether we should mention a „multiregional evolution within Africa”, because the continent is wide enough to contain a large number of separated populations, who could have crossbred periodically or could have been split into other groups (fusion and fission). Same author suggests that the „mosaic anatomy” of some Sapiens skulls could be the consequence (and proof) of the genetic exchange between distinct African populations, some still preserving at those times archaic traits [ibid., p. 256].

In an article from 2016, Stringer reiterates this thesis and argues that human evolution wasn’t a linear progress towards modern day morphology. His opinion is that originally, the proto-sapiens had plesiomorphic traits – which means that they had specific traits belonging to various species of hominins, unrestrained by the specific morphology of a certain evolutionary line. Following the delimitation of the Homo Sapiens species this anatomical complexity was preserved, various populations evolved in an uneven way and there was for millennia a chronological superposition of the “archaic” and “modern” types of Sapiens [Stringer, 2016].

Therefore, along the genes exchange from other hominin species (Neanderthal and Denisovan, at least), the Sapiens themselves were, from the very beginning, an eterogenous group, formed by a mixture of archaic traits. On the African continent coexisted various isolated sub-divisions of the Homo sapiens species, periodically crossbreeding, contributing to the enhancement of the human variation. Similar conclusions presented by other studies, unveil the amazing diversity of the Sapiens in Africa [Gunz et al., 2009]. The Russian anthropologist Alexandr Kozintsev admitted, on his own turn, that natural selection could have contributed to the parallel and simultaneous development of the Sapiens features among several archaic African lines. The contact between them would have consolidated those features; such parallelism is admissible only in Africa, not outside it (as a non-African parallelism is not supported by the existing paleoantropological records) [3].

In other words, the emergence of the Homo sapiens was not limited to a single event. It was the consequence of a long process of crossbreeding and combinations between the proto-sapiens populations, then between Sapiens, evolving in different regions within Africa (and outside it). These populations have interacted and exchanged genes in a way similar to a 3D mosaic pattern, within geographical areas insufficiently delimited for the time being – an African multiregionalism, so to speak. Therefore, in Africa, the transition from proto-sapiens to the anatomically modern Sapiens occurred in an asynchronous way, in several evolutionary hotspots, in South, North, East and West of the continent. However incredible it might seem, the Homo sapiens species had, from the beginning, multiple origins and later on it has welded through the interaction of those original populations. The process of mosaic evolution continued after the spreading of the Sapiens, in ecological niches, across the planet. Next, they mated with Neanderthals, Denisovans and, possibly, other populations of hominins. In this way, through interspecific hybridisation, but also via long isolation in new areas, unequal evolutionary direction has occurred in the primordial heterogeneity of the human populations.

The migrations and spreading of the Sapiens over wide geographical spaces, far apart from each other, in Africa, Eurasia, Oceania and the Americas have catalysed the divergence of these biological traits. Some regions, such as Asia, were populated with Sapiens following several waves, many millennia apart. One such wave occurred roughly 120 000 years ago and another circa 60 000 years ago. According to the existing evidence, the Sapiens of the first wave crossbred with the local hominins, enduring the effects of natural selection in those areas. In this way they’ve acquired new adaptations, inclusively behavioural, which, later on, either disappeared or were amalgamated when they’ve interacted with the Sapiens of the latter migrations [Bae et al., 2017]. It would seem that these later Sapiens have inherited only a small part of the earlier Sapiens genes and hybridised in their own turn with other hominins, then evolved in several branches, populating various ecological niches across Asia, Australia and Europe. For millennia – in some cases as much as 50 000 years – the human populations lived separately, either because of the inborn xenophobia and competition for resources or due to geographical barriers. This is how was created important variety, an enhanced human diversity, which is firmly reflected in specialised studies [Lahr, 2016]. These facts support the assumption that from the beginning, and throughout history, mankind has had a mosaic evolution.

Based on these considerations, I wasn’t surprised by the study proposing that human evolution has a mosaic-like process, published in July 2018 by a group of researchers, amongst them Chris Stringer. All of the latest paleoanthropological data supports such conclusions. In particular, it has become increasingly clear that Homo sapiens emerged after tens and hundreds of millennia of coexistence of several populations; geographically semi-isolated, separated by rivers, mountainous chains and deserts. These populations were acquiring specific adaptations related to the local ecology and climate, which can vary considerably between African regions. Each population was only endowed with some of the common traits of the Sapiens, but those early Sapiens managed to mate sporadically, and in this manner, they’ve contributed to the transfer and sharing of features and behavioural practices – a process of reintegration which, eventually, led to the emergence of a definitive and unitary species some 100 000 to 40 000 years ago. The authors of the study are supporting their conclusion that there wasn’t a single origin of Sapiens but there were several population subdivisions, interconnected, intermittently mingling. This polycentric theory can explain the diversity of sapiens fossils and skulls, located all over Africa, in places such as Jebel Irhoud in the North, Omo Kibish and Herto in the East and Florisbad in the South. This pattern can reconcile contradictory interpretations of early Homo sapiens fossils varying greatly in shape and scattered all over Africa. This pattern also explains the genetic diversity of the contemporary African populations, which exhibit deep archaic separations. The conclusion of the study is that human evolution occurred as gradually (in a Darwinist manner) as well as in leaps (following the way of the punctuated equilibrium) [Scerri et al., 2018]. The study’s arguments are complementary and similar to the explications given above in this text, which is proof that the hypothesis of a mosaic evolution of human species is now ripe for the public.

The concept of mosaic evolution seems to be a compromise in the dispute between mono- and polycentric views on human evolution; in fact, the human species developed its modern traits in several anthropogenic hotbeds, but the origin is African.  Homo sapiens are a mosaic species. In addition, if we were to relate to the Darwinian classical metaphor of the tree – then we must admit that it wouldn’t be a typical tree, such as an oak, which has a single trunk and branches. It would be more like a banyan, an Indian tree, notorious for its astounding ramification, which forms a thicket growing not only vertically but also horizontally. Its multiple roots and branches are intertwined and mutually supportive, there is no main trunk but several autonomous trunks, equal in size, which, altogether are part of the same organism, stretching over a considerable area. Both metaphors, mosaic and intertwined tree trunks, are pointing towards the idea that our species has multiple origins. Likewise, both metaphors are suggesting that the human diversityappeared inclusively through hybridization with related species.

Consequently, the geographical isolation and genetic introgressions with other hominins have laid the foundations of the mosaic micro-evolution of the Sapiens and its diverse populations. No isolation was long enough to produce a human subspecies. The genetic exchange between populations, however minor, has insured the rather homogenous character of the Homo sapiens. Chris Stringer has issued an idea very similar to these considerations: „‘Modernity’ was not a package that had a unique African origin in one time, place and population, but was a composite whose elements appeared at different times and places, and were then gradually assembled to assume the form we recognize today” [Stringer, 2012, p. 263].

In order to conclude, the human species was from the very beginning a hybrid one. It has resulted from the crossbreeding of two or more populations of hominins, spread across an area which cannot be clearly delimited yet, still sufficiently compact so that the populations could interact during the initial stages of the anthropogenesis. The first populations of proto-sapiens emerged, probably, from Homo antecessor or Homo heidelbergensis and have continued to “acquire” genetic information from other hominins encountered. Subsequently, the Sapiens populations, which have continuously engaged in distinct adaptive processes under the environmental pressure, have developed various physiologic and morphologic features at different evolutionary speeds, yet have managed to preserve, over the millennia, the common features of the human species. This is how the human mosaic emerged.

Author: Dorian Furtuna,
evolutionary biologist, ethologist,
PhD in Biology

References:

Image: Cover from my book on human evolution, in roumanian – „ The Human Mosaic. The Evolution of Humans and the Origins of the Races” („Mozaicul Uman: Evoluția omului și originea raselor”). Painter: Eduard Olaru.

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