Modern Humans: Their African Origin and Global Dispersal

Book Preface

As Charles Darwin observed in 1871, humans are “the most dominant animal that has ever appeared on the earth.”1 Their dominance is measured not by physical size or by numbers or total biomass, although the latter two are impressive. Rather, it is measured by their control of the environment. To begin with, humans have occupied virtually every terrestrial habitat on Earth, which is remarkable for a single species, especially because it was largely accomplished more than 45,000 years ago, when the direct ancestors of living humans first spread across much of the land surface of the planet. Moreover, by manipulating both its biotic and its abiotic components, humans have radically altered the environment to suit their own needs, exponentially increasing their numbers and suppressing or eliminating other life-forms along the way. Here, too, the process began more than 45,000 years ago, long before the emergence of the earliest civilizations. Humans’ control of the environment is based on their ability to manipulate objects and materials in complex ways (that is, to make and use complex artifacts). There is nothing remarkable about the capacity to move or modify physical objects and materials, which is widespread in the animal kingdom, but the level of complexity that underlies human technology is unique. Humans can translate a large body of information stored in the brain into a hierarchically organized “artifact” such as a set of winter clothing or a sailboat. The artifacts of humans may be designed with an autonomous function—a machine, such as a self-acting rabbit snare or a mechanical clock—that Thomas Hobbes described as “artificiall life” in 1651.2
Evolution: The Major Transitions
The evolution of life on Earth, from its beginning more than 3 billion years ago, provides a perspective on the unique ability of humans to translate information from the brain to structure—including functioning structure—in the form of a complex artifact. As John Maynard Smith and Eörs Szathmáry observed, the evolution of living systems has been characterized by a series of “major transitions.” Each reflects a fundamental change in how information is stored, transmitted, and translated. An example is the transition from single-celled to multicellular organisms, which took place more than 1 billion years ago. And each transition represents a quantum jump to a new level of complexity with emergent properties.

Maynard Smith and Szathmáry included humans on their list of major transitions in evolution.4 Humans store, transmit, and translate information in novel ways; moreover, the changes in information represented by humans pertain to information in the brain (or neuronal information) rather than information in the genes. Humans store and transmit neuronal information in the form of language. In fact, the units of language, which include sounds made with the vocal tract, as well as the imagined sounds of the vocal tract reproduced in the brain, are translated from neuronal information. They, too, may be considered a “complex artifact,” even when they are not being rendered in material form (for example, written words).5

And the units of language may be manipulated in hierarchically organized structures with many levels and subcomponents, analogous to a machine (or what Daniel Dennett described as a “virtual machine” in the head).6

The capacity of humans to translate neuronal information into material structure, including functioning structure, represents a major transition in evolution, and it parallels the translation of genetic information into functioning structure (living organisms). Some suggest that the earliest lifeforms were both information and structure or, more specifically, RNA acting as an enzyme (or a ribozyme).7

Even the simplest cells (prokaryotes) require the translation of genetic information into proteins and other functioning structures. True cells, or eukaryotes, which may have evolved as early as 2 billion years ago, store and transmit a much larger quantity of genetic information than do prokaryotes. They became the basis for multicellular organisms and the metazoa, including vertebrates, which undergo a protracted process of development from a single fertilized eukaryotic cell that entails the translation of massive amounts of genetic information into functioning structure (figure 1.1).8

Evolution as Computation
One way to look at evolution is as a form of computation. The evolutionary process contains all the basic elements of a computation: “input” (random changes and/or recombination of genetic information that is translated into organisms), “operations” or “defining functions” (natural selection of organisms), and “output” (changes in the genetic information and the organisms).9
At least some of the changes produce an organism that is better adapted (or exhibits a better “fit”) to its environment. And because the input reflects random events, evolution is a “nondeterministic”—even creative—computational process, constantly yielding what Darwin described as “endless forms most beautiful.”10 The immense variety is a consequence of the many hierarchical levels of living systems.
The computations of the evolutionary process take place on several levels, however, and with more than one form of information.11 Each living species is a product of computation with genetic information that occurs on the level of the evolving lineage: the information and the organism evolve together—neither can evolve without the other—over time. Thanks to recent and remarkable developments in the recovery and analysis of ancient DNA (aDNA), we have a partial “fossil” record of both the information and the organismal structure to which it is translated (fossil plant and animal remains) in the course of development (figure 1.2a). Long ago, prokaryotes evolved a form of computation on the level of the organism that allows an individual to respond to unpredictable variations in its environment. Proteins in the cell wall of a prokaryote transmit chemical signals—a simple non-genetic form of information—about the presence of potential “food” sources or threats, and the organism responds by moving toward or away from whatever triggered the signal. Unlike the computations of an evolving lineage, the process is a deterministic one with a predictable outcome.12

The metazoa evolved a much more complex form of computation on the level of the organism, with a new type of information generated at the cellular level. Specialized eukaryotic nerve cells, or neurons, transmit and store information in the electrochemically charged structures (synapses) that connect one neuron with others. And, like the mutation and recombination of genetic information, metazoan development includes a randomizing process that renders computation with neuronal information nondeterministic and potentially creative (figure 1.2b).

Content

Chapter One
Information, Complexity, and Human Evolution 1
Chapter Two
Modern Human Origins and Dispersal: The Synthesis 47
Chapter Three
An Evolutionary Context for Homo sapiens 110
Chapter Four
Recent African Origin 146
Chapter Five
Global Dispersal: Southern Asia and Australia 207
Chapter Six
Global Dispersal: Northern Eurasia 248
Chapter Seven
Global Dispersal: Beringia and the Americas 302
NOTES 349
BIBLIOGRAPHY 423
ACKNOWLEDGMENTS 483
INDEX 485
C