For a century and a half, scientists have struggled to construct the story of the human past. Each new fossil find, or new detail gleaned from an old fossil find, elicits headlines and commentary, followed by the customary call for more evidence before any question can be resolved. It's often said there are as many interpretations of the evidence as there are paleoanthropologists, so it's useful when someone steps forward to take stock of the field and try to discern pattern from static.

In his latest book, Becoming Human, Ian Tattersall distills the evidence for human evolution down to those essential traits that seem to be uniquely human, innovations that we can claim as our own. Although he thinks it's important to understand where we've come from in our evolutionary saga, he believes that our species represents a "totally unprecedented entity" on Earth. "Homo sapiens is not simply an improved version of its ancestors - it's a new concept," argues Tattersall, curator of anthropology at the American Museum of Natural History.

A prolific writer on the topic of human origins, Tattersall makes an erudite and opinionated guide on this journey to discover how we came to be who we are. Like a non-majors college course, Becoming Human is a breezy survey that touches on a broad range of topics but only occasionally delves deeper. Tattersall has dealt with the details of our fossil record in other books, most recently in The Fossil Trail; here he prefers to lay out his ideas of the evolutionary process, natural selection, and speciation to reveal why paleoanthropologists have frequently been misled in their attempts to understand evolution.

In Tattersall's opinion, much of the mudslinging rife in paleoanthropology is due to fundamentally different views of how evolution works. Traditionally and even today, he writes, our evolutionary history has been depicted as "a long, single-minded slog from benightedness to enlightenment," or "from primitiveness to perfection." The problem arises partly from having only ourselves - a single terminal twig on the family tree - around today to study. With only a single species' origin to trace back in time, paleoanthropologists have fallen back on a linear view of evolution that biases readings of the fossil record.

Tattersall maintains that we consistently underestimate the number of species in our own lineage. The simple story of one hominid, an upright human ancestor, evolving into the next in succession can no longer be supported. The family tree in Becoming Human recognizes 16 species for the past 4.5 million years. In taxonomic parlance, Tattersall is a splitter, favoring adding several species to fill out the family tree. For example, he is one of a growing number of experts who recognize two species of early Homo, living about two million years ago, only one of which may have left descendants. Tattersall also reserves the name Homo erectus only for certain fossils from Indonesia and China, preferring to place slightly older African fossils, such as the spectacular skeleton of a teenage boy from Nariokotome, Kenya, in the species Homo ergaster. With remarkably modern-looking anatomy, these "tall, striding savanna beings" were the first to display typically human wanderlust, but ergaster lacks the expanded brain and capacity for culture that characterize our later evolution.

The author's penchant for splitting continues the closer in time we get to our species. Europe presents a textbook case of evolution in isolation. The continent's first occupants, who arrived at least 800,000 years ago, may have been discovered recently at Gran Dolina in Spain. Though Tattersall reserves judgment on the Spanish authorities' claim that the Gran Dolina fossils represent a new species, Homo antecessor, he does favor placing younger European fossils into the species Homo heidelbergensis, which ultimately evolved into the Neanderthals and may also be our own immediate ancestor.

Tattersall's critique of the traditional view of our evolution extends to the process and pace by which new species appear. The author chides his colleagues for being outside the mainstream of evolutionary thinking and for long being lulled by the seductive simplicity of gradual change by natural selection. Instead, he sides with his American Museum of Natural History colleague Niles Eldredge and with Stephen Jay Gould, both of whom have advocated a model for evolutionary change known as punctuated equilibrium. Basically, this idea takes account of obvious gaps in the fossil record of life as evidence that evolution proceeds in fits and starts. New species arise fairly rapidly during times of drastic environmental change, when organisms are forced, essentially, to evolve or die. But throughout longer, more quiescent periods, species may show no sign of evolutionary change.

Tattersall contends that punctuated equilibrium provides a more complete account of our evolution. While some of our earliest ancestors among the australopithecines persisted largely unchanged for up to a million years, there is also evidence for episodic events in our deep history. He disagrees with Eldredge and Gould that new species always arise infrequently. The anatomical and behavioral gulf that we observe between us and our earliest primate relatives must be missing plenty of intermediary forms.

Despite his dislike for linear thinking, Tattersall adopts a chronological approach to recapping the features and achievements of our ancestors. Evolution happens when innovations arise within a species. But besides our bipedal posture, argues Tattersall, we don't have to dig very deep into our past to find what makes us uniquely human. It's what resides behind our foreheads, and how we use our brains to construct and remodel our social and physical worlds. Our brains grew remarkably large during the previous two million years, but human uniqueness may depend ultimately on some minor, internal reworking that permitted consciousness, which is both our most conspicuous and elusive asset as a species.

Our other unique attributes, specifically symbolic language, stem from our same cognitive capacity to acquire, retrieve, and create information and to represent the world in novel ways. Symbolism, whether the colorful designs and animals painted in caves such as Lascaux and Chauvet, or the more subtle engravings of Combarelles that Tattersall evocatively describes in the prologue, "lies at the very heart of what it means to be human," and it appears only within the last 50,000 years of our existence.

Because our brain is the product of evolutionary history, according to the currently fashionable field of evolutionary psychology, a firm genetic grounding must constrain our behavior. Tattersall, however, regards this perspective as "woefully wrong." As with the rest of our evolutionary story, modern behavior is more complicated, and too intricate to be guided by genes alone.

Several recent books have endeavored to explain consciousness or the evolution of language and the human mind. By comparison, Tattersall's treatment of these subjects is superficial and ultimately unsatisfying. He speculates, for instance, that speech and language developed among modern humans as an exaptation, an evolutionary by-product, of particular throat and cranial anatomy that developed for another purpose, but one wishes for a fuller explanation of what amounts to the crux of humanness. In the end, we are left to ponder our past, present, and future as "idiosyncratic, unfathomable, and interesting creatures," who continue to write and read books that might resolve a bit more of our mysterious nature.

Brain evolution has not proceeded by the simple addition of a few more connections here and there, finally adding up, over the eons, to a large and magnificently burnished machine. Opportunistic evolution has conscripted old parts of the brain to new functions in a rather untidy fashion, and new structures have been added and old ones enlarged in a rather haphazard way.