Exciting new studies in human evolution are appearing rapidly, transforming scientific understanding of how the human community took form. A 2018 article by archaeologist Eleanor Scerri and colleagues, for example, identifies key debates on this topic. They ask: When and how did Homo sapiens become a species? How important were subgroups and migration in human evolution? And while Scerri cannot yet propose a specific date or place for the origin of Homo sapiens, she reveals certain misunderstandings in earlier thinking about human populations, then points to new directions in interpretation.
Scerri argues that paleontology shows varied physical populations with varied material culture, geographically spread through Africa. She notes genetic evidence suggesting that the lineage for Homo sapiens traces back to 500 ka (“ka,” in this essay, means “thousand years ago”); she also suggests human admixture with other hominin populations in Africa.
Physical remains of humans are now labeled as Homo sapiens from 300 ka. The ecological record suggests shifts in African landscapes over time, and present-day African populations continue to reflect ancient variations. Underlying these patterns, Scerri argues, were “structured” (i.e., subdivided) human populations throughout Africa, rather than an initially small, uniform, and localized population. The key to an improved interpretation of human evolution, she says, is more fully specified models of population structure.
Scerri’s argument on “structured” populations in Africa, while important, is just one issue in the broad debate over human evolution. Many other questions remain. From what ancestors did they come? And more broadly, what types of information do we need to understand such aspects of human evolution? It is remarkable how much has been learned—and how much remains to be learned.
Assessing the Strategy for Evolutionary Analysis
The purpose of this essay is not only to report on recent evolutionary news but to consider the overall strategy of evolutionary analysis. It’s a review of the categories of evolutionary analysis from the beginning of Homo sapiens to recent times, as seen in debates since the time of Darwin.
After identifying categories of information on evolution; I point to dynamics of change within each of those categories, then point to examples of discoveries in evolution. In conclusion, I suggest unification and differentiation of the human population as two broad evolutionary patterns that combine the many specific types of change. A final note focuses on the role of analytical modeling in developing specific arguments within the context of unification and differentiation.
Categories of Information on Evolution
These six major categories show that early humans led complicated lives and that modern evolutionary scientists have had many issues to consider:
- Phenotype (from the 1870s). This is the study of the physical beings of humankind, their historical remains, and their development and behavior over their life course, including the basic biological structures of household and community.
- Genotype (from the 1980s). Genomics is the study of human DNA and its genes, which evolve and produce proteins that generate all human processes and activities.
- Population (from the 1920s). Total numbers of humans and their subgroups are measured in two ways: as “census populations” of all adults and children and as “effective populations” of breeding females, estimated genetically. Population includes the “speciation” or formation of humans into a coherent and distinctive form of life. Population also includes types of human migration.
- Environment (from the 1950s). The environmental factors of physical geography, climatic change, plus flora and fauna—linked into ecological webs—influence human life but are also changed by human activity.
- Culture 1 (from the 1980s). This is culture as understood and studied by biologists and environmentalists, who call it “social learning.” Social learning describes the ways in which individuals observe others, develop new behaviors, expand their cooperation, and influence their genome. Among humans, this process accelerated about 300 ka and is still influential in human networks.
- Culture 2 (from the 2010s). This is how culture is understood and referred to in today’s world. It may also be called “group culture,” and its connections to biological evolution are just now being analyzed. At its most basic level, group culture began when humans formed conscious “we-groups,” whose decisions created syntactic language and institutions to complete tasks—a process that accelerated about 70 ka. This led later to the creation of much larger social groups such as the businesses and states of today.
Major Evolutionary Dynamics
Each category of evolutionary information has subcategories that change and interact with one another. For each category, here are the major dynamics or processes of change:
- Phenotype: changing physical form, steps in life-course development, group behavior, incorporation of new processes into the human order
- Genotype: DNA mutation and selection, RNA and protein creation, epigenetics and regulation of gene activity
- Population: growth and decline in populations (and subpopulations), displacement of migrants, admixture among migrating groups
- Environment: short- and long-term changes in geology, climate, and flora/fauna—and how those changes influence the categories of human evolution
- Culture 1: individual learning in humans and other species (known as “social learning” and “cultural evolution”), the expansion of cooperative behavior
- Culture 2: group decision-making in humans, group activity in representation or modeling of the world, media of expressive culture
Listing these dynamics is instructive, but in viewing them we must keep in mind that the human genome has only 30,000 genes—not much more than in far simpler animals. Thus, it is the interactions among the genetic and other dynamic processes that paint the full picture of human existence.
Some Key Discoveries
For the categories of evolutionary analysis and the dynamics within them, I note some major discoveries and their influence.
- “Effective population” of breeding females theorized (1931); estimates of early African effective population (1990s)
- Occasional migrations into Arabia from 400 ka, reported in 2017
- Geological observations on long-term African historical climate (after 2000)
- Niche construction theory on interaction among evolutionary categories (1980s)
- Culture 1
- Culture 2
- Hypothesis on adolescent leadership in syntactic language and institutions (2010s)
Understanding Unification and Differentiation
Evolutionary science needs to know more about fluctuations in the variety within human populations. Consider this: Populations with closely similar genomes and phenotypical form and behavior are labeled as “species.” Since the human genome is far more unified than that of other mammalian species, we would seem obviously to be a species. Yet fossils of human populations continue to reveal specific characteristics for every region and local community. Thus, in evolutionary terms, questions remain about when humans have become more similar—or more unified—and when they have differentiated over time.
You can look at the dynamics and discoveries listed above and consider which of them might lead to unification of the human species and which might lead to differentiation. For instance, migration of small and isolated groups leads to differentiation through “genetic drift.” But large-scale migration from one population to another causes the two populations to become more similar. Therefore, processes of both unification and differentiation have taken place throughout human history. Another question to consider: In what situations did either unification or differentiation predominate?
Analytical Modeling of Past Processes
Scerri’s 2018 article makes recommendations for changes in models of genetic change and population structure. To enact such changes, researchers must make explicit and sensible assumptions that define variables and their interactions. This would ensure that the resulting scientific models of unification and diversification can project detailed simulations, which can then be compared to real data.
Fortunately, the science of network analysis has greatly advanced the modeling of large and complex datasets—by including the specific forms of networks and their links to each other. Techniques that are now being applied in medicine, for example, might be modified and applied in the history of evolution. Whatever the application, the modeling will be complex, because it must address the various scales of human existence: those of the genome, cells, human organs, individuals, and the different types of behavior of populations, as well as the environmental influences at each scale. It will be interesting work.
Based on this simplified summary of the strategy of evolutionary analysis, I will return in two later essays to offer narratives of some major steps in human evolution over the past half-million years.