In part one of this series, I examined the six main categories of human evolutionary change and considered how each of them might lead to unification and/or differentiation of the human species.
In part two, I expand on the tale of human evolution, building on models of structure, dynamics, and interactions. I propose plausible hypotheses of growth and change based on models at the levels of phenotype and genome. As I argue, populations of Homo sapiens experienced three concentrated eras of innovation, each followed by a wave of migration and exchange. Interacting pressures of unification and differentiation, fluctuating with climate (shown in Figure 1 below), influenced successive populations in Africa. The resulting picture of human evolutionary transformations can be tested and revised based on expanding empirical observations
Phase 1: Homo sapiens as a species, ca. 500 ka
Biological processes of natural selection—genomic and epigenetic change—guided the speciation of humans. At perhaps 500 ka, in a warm and humid period (see Figure 1), there emerged ancestors of Homo sapiens whom we can can call “humans.” Under these changing circumstances, genetic mutations and epigenetic or developmental changes led in several directions, bringing phenotypical changes. In a genetic mutation, the Merge capability advanced internal human logic, enabling a change in behavior in which people could lump concepts together and link them to new concepts. Phenotypical changes included expanded brain size, reduced face size, lighter skeleton, and smaller teeth. Community size also expanded, eventually reaching an average of 150 members. Yet Acheulean stone technology changed little in the these early days of Homo sapiens.
In intimate groups, developmental change caused the polygamous family structure to be replaced by pair-bonded households as the locus of eating, sleeping, household tasks, and the nurture of offspring. Polygamy had left females subordinated and many males without mates, while the new male-female teams supported female collaboration, so that larger numbers of offspring survived. This encouraged population growth and provided more labor for community tasks.
These community changes gave rise to the overall process of human speciation. Today, biological species are still defined as populations with broad similarities at both phenotypical and genetic levels. Yet details of the definition continue to evolve. For example, while Darwinian thinking initially treated species as unified populations, new evidence on genetics and environments has revealed complications. For humans, it now seems more likely that separated communities connected and exchanged innovations via migration—thus beginning as a diverse species derived from different ancestral communities but with growing similarities.
Phase 2: Initial diffusion, up to 300 ka
In another aspect of biological evolution, the first great migration of Homo sapiens began as humans moved within their highland homelands and then left for places where humans had hardly been before.
Figure 2 reflects genetic movement, arguably confirming distinctive human communities in most African regions as far back as 300 ka. Migrants adapted their habits to each locale but also transformed their new homes through “niche construction.” Bodies of Homo sapiens gradually changed in response to regional environments: forest dwellers became smaller, while settlers in northern Africa developed lighter skins to absorb adequate sunshine. Climate sometimes became cold and dry, forcing further migrations and even population decline. Yet overall, by 300 ka, human populations had expanded continent-wide—and their shared characteristics were far different from those of surviving erectus, heidelbergensis, or other hominin groups.
Phase 3: Middle Stone Age (MSA), ca. 300 ka
A process of cultural evolution gained significance among humans about 300 ka, according to models in the fields of ontogeny and dual inheritance. Kin selection and social learning facilitated tool-making; communication emerged through emulation, gesture, dance, and protolanguage (in which small vocabularies of isolated words were shared within local networks). These cultural advances accelerated epigenetic development and other processes of biological evolution.
Archaeologists identified this era as the Middle Stone Age because of such new technology as prepared-core manufacture of stone tools, hafted spears, and intensive use of fire. Further, paleontologists have confirmed dates for skeletal remains that fit with the same era—applying the term Homo sapiens to three distinct skulls: from Jebel Irhoud in Morocco (315 ka), Kabwe in Zambia (299 ka), and Omo Kibish 1 in Ethiopia (230 ka). Figure 3 shows these skulls, along with those from other lineages. Meanwhile, household nurturing of offspring contributed labor that expanded community activities. Thus, hunting and toolmaking relied more on community than households.
Phase 4: Migratory exchange of innovations, 300–100 ka
The era of biological and cultural evolution, relying on technical and cultural innovations of the Middle Stone Age, brought a second great migration throughout Africa and even beyond. Households produced larger numbers of maturing offspring; larger numbers took up community labors including tool-making, firewood collection, and participating in communication. In the exchanges of this era, the human skull became increasingly globular, and the chin emerged—though each pattern leaves evidence of human diversity.
Eras of high humidity brought not only expanded settlement in the Sahara but also periodic settlements in Arabia. The Arabian deposits of steadily evolving stone tools since 400 ka (Figure 4) reveal migrations by humans who diffused in humid times, then withdrew to Africa as drought drove them out. Parallel migrations may have spread throughout Africa, including cases where settlers joined existing populations. The Arabian examples indicate the overall magnitude of the second great African migration, the fluctuations of drought and humidity, and the development and sharing of innovations across the continent.
Phase 5: Syntactic language, 70–65 ka
Syntactic language took form in a single region and spread to all humans, thereby creating the broader structures of institutions and social evolution. Social evolution was distinctive because of the new ability to form self-conscious groups, but social evolution also interconnected tightly with the ongoing processes of biological and cultural evolution.
The innovation emerged east of Lake Victoria, where archaeology documents the finely made Later Stone Age artifacts from 65 ka. I hypothesize that “we-groups” of adolescents played and practiced until they had created and shared common words and syntax for combining them into complete sentences, not just isolated words. Their skill in categorization, based on a verbal application of their Merge capability, shaped the logic of full sentences.
The creation of language required years of devotion by bright and maturing adolescents, debating and working together to agree on common speech patterns. It spread by teaching to other adolescents and some adults—then to adolescents in other communities. Siblings and parents began to teach language to the toddlers in their households. Languages, spoken in communities of 150 people, became institutions governed by common agreement. Communication by speech led to the advance of the Later Stone Age.
Phase 6: Language in Africa; settlement in Eurasia and Oceania, 65–25 ka
Social evolution led to the third great migration of humans. The migration was distinctive in that spoken language came from a single origin (though languages, once formed, evolved further with time and in every place). Within Africa, speaking migrants settled among non-speaking human populations and introduced language—parallel to the MSA exchange of fire and hafted spears. Beyond Africa, speaking migrants settled new lands, encountering the hominin lineages of Neanderthals and Denisovans—parallel to the first wave of human migration. Both the African and Eurasian migrations can be documented through genetic, archaeological, and language-distribution evidence.
By 25 ka, structured populations of Homo sapiens had passed through great waves of unification and diversification, innovation and migration, expansion and transformation. Many details are yet to be filled in, but this framework—relying on migration of diverse and structured populations but anchored in households and community groups—enabled humans to undergo immense changes within half a million years. With the Ice Age of the Last Glacial Maximum, peaking from 25 to 18 ka, a fourth set of innovations opened Phase 7 of human evolution—triggering changes that transformed Homo sapiens once again, leading to the human society we live in today.