Every family has its genealogical myths, legends and secrets. There’s the Native American ancestor some clans like to talk about and the Jewish or black (or in the case of African-American families, white) great-great-grandparent that no one mentions or even knows for sure existed. Whole nations tell themselves similar stories about the past. Icelanders believe their country was settled by Norsemen and the British or Irish women they brought (often unwillingly) with them. British schoolchildren are taught that when the Anglo-Saxons invaded Britain in the fifth century, they pushed Britain’s Celtic inhabitants out to the hinterlands of Scotland and Wales and made England an essentially Anglo-Saxon country.

Until recently, it’s been impossible to prove or disprove any of these stories. DNA analysis has changed all that, and as New York Times science reporter Nicholas Wade explains in his new book, “Before the Dawn: Recovering the Lost History of Our Ancestors,” in the process it has toppled more than one cherished belief. It turns out, for example, that most Icelanders are probably descended from Norsewomen and that a large proportion of the male population of Britain carries the Y chromosome of the Celtic speakers who were supposedly chased off the land by the Anglo-Saxons. Similar research has established that an astonishing 8 percent of the men living in the vast territory formerly controlled by the Mongol Empire are most likely direct descendants of Genghis Khan.

The power of DNA analysis to nab criminals, exonerate the wrongly convicted and determine a baby’s true paternity has understandably impressed everyone and provided new fodder for trashy daytime talk shows. With “Before the Dawn,” however, Wade goes further, offering a survey of how cutting-edge genetics has been combined with a variety of other sciences to solve, or at least further illuminate, some long-standing puzzles in humanity’s distant — and more recent — past. As with any powerful new technology, it’s easy to get carried away. (My own enthusiasm prompted me to send a sample of my DNA off to one of several new services that offer ancestral DNA analysis, hoping to learn something about the murkier corners of my own genetic heritage, but more on that later.) It’s easy, in other words, to think that because DNA can tell us so much, it can tell us just about anything, and that would be a dangerous mistake indeed.

Here, greatly simplified, is how it works: When sperm and egg combine to create the embryo of a new organism, a whole lot of swapping of genetic code goes on. The DNA that spells you is a salad of code taken from both your mother and your father. Their DNA is a mix of each of their own parents, and so on back into time immemorial. However, a big portion of the DNA in each of our cells isn’t used to make our bodies; this is what’s often called “junk” or “filler” DNA. Both the active and the filler DNA are subject to small, random variations when the code is transferred from one generation to the next; that’s mutation.

The DNA that’s actively involved in making us what we are is subject to “selective pressure.” If a new mutation makes a human being who’s a little bit taller or faster and that change makes the individual more successful in his or her given environment or more attractive to the opposite sex, then that individual will be more likely to survive and to reproduce more plentifully. Over generations, the mutated gene will become more common, a process called natural selection, which (we can only hope) most of us learned about in school.

Filler DNA, however, because it has no input in shaping the physical organism that carries it, isn’t affected by the natural selection process. It still undergoes the occasional mutation, but those mutations simply accumulate over time without doing anything. In particular, two sections of this DNA — the Y chromosome, which men hand down to their sons, and mitochondrial DNA, which women hand down to all their offspring — have been useful to those researching human ancestry. If, say, 10,000 years ago a particular woman was born with a certain mutation in her mitochondrial DNA, all of her children will carry the same mutation in their mitochondrial DNA, and her female children will pass it on to their children. The same is true for a man of the same period born with a mutation in his Y chromosome, although he will only pass it on to his male offspring, who will only pass it on to their male offspring, etc.

By looking at the Y chromosomes and mitochondrial DNA of various populations in various places, scientists can get a good idea of where these genetic lineages began and how they spread over the globe. The most prominent popularizer of this notion is the Oxford geneticist Bryan Sykes, author of “The Seven Daughters of Eve,” in which he employed the fairly kitschy device of naming the seven women from whom all individuals of European origin are descended, and describing their lives as he imagined them. Yes, it sounds very “Clan of the Cave Bear,” but at its heart there’s an intriguing fact: All contemporary humans are part of one of 38 major “haplogroups,” each founded by a single woman.

A few of those lineages stayed in Africa, where human life originated, but everyone else on the planet, Wade reports, is descended from a tiny group of people who migrated out of Africa 50,000 years ago. That group might have consisted of as few as 150 hunter-gatherers, but eventually their descendants spread as far as Australia and North America to populate the world. Researchers lean toward the theory that there was only one migration out of Africa, one little band of people who braved either the coastal edges of the Arabian desert peninsula or the perils of the eastern Mediterranean, which was then occupied by fierce Neanderthal tribes.

The idea of this exodus is irresistibly romantic, but it can’t be said that Wade takes advantage of that. “Before the Dawn” is at best a workmanly account of what is, after all, a field rife with controversies, reversals, ambiguities and political land mines. Still, what the book lacks in vividness it doesn’t exactly make up for in clarity. The descriptions of how DNA works are more difficult to understand than they should be (although the processes are fundamentally hard to grasp). Genetics, unlike, say, quantum physics, is new enough that it hasn’t yet found a great, nonpolemical popular science writer to do it justice.

Nevertheless, Wade has collected many fascinating stories about the creativity and (occasional) hubris of those scientists who have used DNA analysis to study the past. When did human beings first begin to wear clothes? That used to be impossible to estimate, since the clothes themselves have long since crumbled to dust. But when Mark Stoneking, a researcher at the Max Planck Institute for Evolutionary Anthropology in Leipzig, got a notice from his kid’s school about a problem with lice, he had a brainstorm. Body lice, unlike the head lice from which they evolved, have claws “specialized for living in clothing, not grasping the shafts of hair.” Since the lice can’t live for more than 24 hours away from the warmth of a human body, they must have evolved shortly after human beings (whose hairiness has drastically decreased over the millenniums) began to wear clothes. Comparing the DNA of head lice to body lice gave Stoneking a date for when body lice emerged.

But there’s a catch. The kind of analysis Stoneking performed provides only a ballpark figure: 42,000 to 72,000 years ago. That 30,000-year spread may not be much on the scale of the history of life itself, but in human terms, it’s pretty wide. Did this happen before or after our ancestors left Africa, a date estimated at 50,000 years ago? Your guess is as good as mine.

Wade, who is definitely a worshipper at the church of genetic determinism, usually gives equal time to those who offer objections or counterexplanations for historical changes in human behavior, but his preference for genetic theories is plain. The problem is that there is so much ambiguity and wiggle room, so much that is unknown and as yet unprovable about a lot of these questions, that many of the theories touted in “Before the Dawn” are unconvincing.

The more recent the history in question, the more developed the human society you’re talking about, the less possible it becomes to tease out the genetic causes from the cultural ones. As Wade himself points out, culture began to shape genetics at least as early as human beings began to domesticate cattle. Lactose tolerance, the ability of adult humans to digest cow’s milk, is today found in the highest concentration in the regions once occupied by the cultures that first domesticated cattle. So, while it’s easy to credit Wade’s favored theories of early modern human development — that evolution lies behind the transformation from aggressive and warlike hunter-gatherer cultures to more peaceful, settled communities, for example — at a certain point it sure looks like culture started to move faster and more decisively than evolution.

The most self-consciously controversial chapters in “Before the Dawn” concern race and gender, specifically recent efforts to find the genetic significance of both factors. Wade and others of his inclination are quick to blame any criticism of such research on a lily-livered obeisance to p.c. dictates. To some extent, they’re right, since it’s obvious to anyone with any common sense that there are some organic differences between the genders and the races.

Nevertheless, by wrapping themselves in the authority of science, sociobiologists have a tendency to gloss over large patches of their own ignorance. Evolutionary psychologists, for example, have a tendency to blithely theorize about human sexuality when they clearly don’t know much about how human beings really behave sexually. True, there’s not much solid data to be had. It’s difficult to do reliable research on the sexual behavior, both because people tend to lie about it and because institutions (especially the U.S. government) don’t want to get mixed up in such “disreputable” projects.

An example of how comically wrong sociobiologists can go when they don’t realize what they don’t know about sex turns up in “Before the Dawn.” Geneticists testing for hereditary diseases, Wade explains, sometimes discover “nonpaternity cases,” that is, children for whom “the father of record cannot be the biological parent.” Geneticists estimate that as many as 10 percent of American children are not the biological offspring of their purported fathers. This is “surprising,” Wade observes, “in light of the control that women now exert over their reproductive behavior.” In fact, he concludes, it can only be that many of these nonpaternity cases are “deliberate,” an example of women taking advantage of the evolutionary principle of “sperm competition,” by getting inseminated by more than one man at a time.

By this line of reasoning, people with access to birth control rarely get pregnant unintentionally, and women only have unprotected sex with men in order to conceive their children. Do I really need to explain that it’s quite common for women, married and single, to have sex without protection, either because it happens spontaneously or, mostly likely, because they’d prefer to believe that it did? Not to you perhaps, but apparently evolutionary psychologists, despite paying “particular attention to human mating habits,” still haven’t gotten the memo. In instances like these, you have to suspect that the field holds a special attraction for those who are made uncomfortable by the complexity of human behavior.

Strangely, this is the only point in the book where Wade considers contraception, perhaps the most flagrant, if recent, example of how human culture — our technology and our ability to communicate — has interfered with the usual workings of natural selection. In contrast to all evolutionary formulas, the most “successful” members of our species (in terms of health, wealth and status) are now producing fewer offspring. Those kids are more likely to thrive, it’s true, but they’re also more likely to lead to a genetic dead end by not reproducing at all, putting the kibosh on some very selfish genes.

Actually, the evolutionary fix was in as early as the introduction of agriculture. Farming is a survival skill that, recent genetic analysis shows, didn’t allow one population of human beings to obliterate another. Instead, agriculture — an idea and a technology — spread from one population to another, giving every human community that adopted it a powerful edge. Our ability to communicate such ideas advances at blinding speed compared to the slow grinding of natural selection.

Much of the cross-disciplinary theorizing Wade describes in “Before the Dawn” is fascinating, but speculative. Currently in the news and explored in Wade’s book is a recent investigation of the possibility that Ashkenazi Jews might have acquired their susceptibility to certain hereditary diseases as a side effect of an equally hereditary propensity toward high intelligence. As Wade puts it, “The suggestion that one group of people may be genetically more intelligent than another is a sensitive subject, not least because it opens the door to the argument that if some groups are smarter, others may be less so.” The hypothesis is that because European Jews were confined to certain very limited and intellectually demanding professions for around a thousand years, intense selective pressure made this genetically isolated population more “intelligent” (by one standard) via mutations that also made them more vulnerable to some diseases.

Should such research be squelched out of fears that investigations into the possibly genetic superiority of one group will endorse ideologies that insist on the genetic inferiority of others? If so, then would we also argue that, say, the overrepresentation of West Africans in the upper ranks of sports that require sprinting is purely a coincidence or a “cultural construct”? And if you would agree with that, then would you also advocate ignoring the fact that some hereditary diseases strike some races more often than others and that some drugs help some races more than others? That, Wade writes, was the case with a drug called BiDil, which was found to work for heart patients of African ancestry far more than it did others.

“To falter in scientific inquiry would be a retreat into darkness,” Wade writes. Maybe so, but in the past, in matters of race and sex, what presented itself as scientific inquiry has sometimes turned out to be a headlong leap into darkness. A little skepticism seems justified. Human beings are exquisitely suggestible and scientists are themselves only human, so it would probably help if they were willing to admit, for example, that their definition of “intelligence” might be a tad limited.

It also helps to remember that highly speculative genetic theories, however confidently advanced, are embarrassingly subject to reversals. In “Before the Dawn,” Wade describes genetic research that supposedly proves that “Jewish women, unlike Jewish men, do not all come from the same ancestral population,” contrary to long-held Ashkenazi Jewish folk beliefs that the communities of the Jewish Diaspora were established by couples, rather than single Jewish men who married women from local populations. Then, just this January, Wade reported for the New York Times on another set of researchers who have come to exactly the opposite conclusion, finding all Ashkenazi women to be descended from four female Jewish ancestors. The researcher who advanced the original theory, of course, has refused to retract his conclusion.

Why, given all this healthy skepticism about what we can learn from DNA, did I still opt to send off a cheek swab to Family Tree DNA and receive an analysis of some of my own “filler DNA” at the cost of 189 bucks? Except for my mother’s brother, no one on either side of my family has taken much active interest in our genealogy, so unlike a lot of FTDNA’s customers, I’m not trying to fill in any stubborn blanks in a lush family tree. In a way, the preponderance of blanks has made me more curious. (Though not enough so, I guess, to actually do any genealogical heavy lifting.) I suspect that even the most hardcore social constructionist believes deep down that some of who we are is in our blood, so how could I not at least wonder?

Specifically, though, there’s my mother’s late father, a Catholic, with an unusual, vaguely French name that my relatives vaguely believe had been changed from something else. After seeing an old photograph of him with his brother, I became attached to the idea that they looked Jewish. Since my uncle hadn’t been able to find out much about their family, partly due to the changed name, maybe a DNA test would tell me more?

Like a lot of DNA test customers, I suspect, I was in for some disappointment. As a woman, I only have one set of genetic markers to test, my mitochondrial DNA. (Men can have both their Y chromosome and their mitochondrial DNA tested. “That’s not fair!” a perpetually aggrieved friend of mine said when I told her about it; I assured her that science could not be blamed for this one.) More to the point, my maternal grandfather’s contribution to my mother’s genes couldn’t be detected by this test, either. I could learn about my mother’s mother’s mother’s mother and so on back to the late Pleistocene era, but nothing that crossed from one gender to the other. Likewise, although my brother can track our father’s father’s father’s father and so on, our father’s maternal ancestors must remain a mystery, at least to DNA analysis.

The informational materials provided by FTDNA to its customers are impressively dry, a daunting mass of charts, numbers and capital letters that yield only a dribble of juice under only the most extreme pressure. The real value of the service — and others like it — is in its ability to connect amateur genealogists who’ve hit a dead end with possible “cousins.” Through FTDNA you can enter your stats into MitoSearch, an international public database, to find more matches. And you can enter your information anonymously into the databases of a project being conducted by National Geographic to track the ancient history of humankind’s migration across the earth.

I learned that I belong to haplogroup H, a vast lineage to which almost half of all Europeans belong, ranging from Turkey to the United Kingdom. This was noted on a certificate I received, along with a list of my “mutations” — that is, the differences between my mitochondrial DNA and that of an (unnamed) European individual whose DNA is used as a standard, the Cambridge Reference Sequence. Ancestral DNA tests list only those differences in two sections of filler DNA known for their propensity to mutation and therefore for their usefulness as genetic markers.

As goofy as the certificate is, there was something intriguing about seeing that list of numbers (73G, 263G, 309.1C, etc.), like remnants of the Dewey Decimal System, that held, coded within them, the history of a chain of women going back tens of thousands of years. One of these women, some 1,000 generations back, possibly lived in what we now call Spain, and her descendants traveled gradually up the Western coast of Europe, following the retreating glaciers of the last Ice Age, until at some point one of them crossed the English Channel and eventually settled in Scotland, where her daughter’s daughter’s daughter, etc., would give birth to a woman named Mary Cummings, who sometime in the 19th century would emigrate to Prince Edward Island in Canada and become the great-great-grandmother of my grandmother, also named Mary.

I’m not saying that Family Tree DNA helped me picture all this — they’re not much help in that department, but the forums where their customers try to make sense of the results they’ve received and swap tips with other haplogroup members are another matter. That’s where you’ll find the human face of ancestral DNA analysis, where people say encouraging things like “Go T2!” and where the number “16519C” becomes a bond between countless strangers. Fellow haplogroup members call each other “cousin.” People post their mutation lists and, as with all good boards, a few kindly and knowledgeable old hands are usually around to explain the hard parts — which with DNA analysis is pretty much all of the parts.

People turn to this kind of DNA test to see if they can verify rumored Native American ancestry (not unless it’s on the direct matrilineal or patrilineal lines, as one member found to his dismay). African-Americans want to know which part of the mother continent their people were stolen from. Other seekers are adoptees who want to find out about their biological heritage and can’t get information from the authorities. Most poignantly, one member described having been “abandoned at birth.” I’m not sure if a DNA test can knit the gap something like that leaves in a person’s life, but I can understand why it might seem worth trying.

As much as I believe that “families of choice” are as good as any other kind and that people shouldn’t be defined by the past, blood does call out to us. The intricate marvel of DNA is more dazzling than any alleged celestial clockmaker, and the idea that all of this around us — from the skyscrapers of Manhattan and supercolliders to “King Lear” and the Great Wall of China — began with just 150 tough, desperate (and possibly clothed) people deciding to strike out across the Red Sea for parts unknown never ceases to amaze and impress me. Maybe not enough to make a genealogist out of me, but then it turns out I do have one match, with, of all things, a woman named Maria Marana, born in Genoa, Italy, around 1820, and that might be worth checking out.

Fish, without a doubt, gotta swim, but how do they do it? And how, over millenniums of evolution, did they get to be so good at it? These two questions have driven the career of John Long, a professor of biology and cognitive science at Vassar College. Long is so into fish that his primal scene of intellectual seduction involved a Ph.D. trying to get him to join her team by taking him out for coffee and asking, “Have you seen the vertebral column of a marlin?” Thus was Long launched into a course of study that would ultimately lead him to the improbable task of making robot fish.

The Miss USA pageant crowned its annual winner on Sunday, but the contest is drawing new attention  for a video of all 51 contestants wrestling with the question, “Should evolution be taught in schools?” The results, as you might expect, are all over the place. To wit: While only a couple said a definitive “no,” dozens more squirmed through answers — trying as hard as possible not to offend anyone — before arriving at the common conclusion that evolution should be taught alongside “alternative beliefs.”

For a new study set to be published in the journal Evolution, scientists from the Center for Functional and Evolutionary Ecology in Montpellier, France, mated female brine shrimp (“sea monkeys”) with males from past and future generations.

The report, called “Male-Female Coevolution in the Wild: Evidence from a Time Series in Artemia Franciscana,” found that the female brine shrimp “survived better and had longer interbrood intervals when mated with their contemporary males compared to when mated with males from the future or the past.” Its formal conclusion: “[T]he process of male-female coevolution, previously revealed by experimental evolution in laboratory artificial conditions, can occur in nature on a short evolutionary time scale.”

Chris Christie, a wealthy, well-educated lawyer from New Jersey, is suddenly not willing to say whether or not he believes in biological evolution. Christie went to a very good public high school and he’s a mainstream American Catholic, not an evangelical Protestant, so I am going to guess that he does believe in evolution, if he ever even gives the idiotic question any thought. I’d also guess that believing in evolution is not particularly controversial among New Jersey Republicans, who are not exactly Kansas Republicans.

Let’s be honest, a kiss is never just a kiss. It is the ultimate romantic symbol in our culture — from Shakespearean tragedies to Gustav Klimt’s gilded embrace to the legendary V-J Day smooch in Times Square to those critical words “you may kiss the bride.” Sometimes it’s instead an expression of affection, elation, loyalty or, on the other hand, disloyalty (see: the kiss of Judas). In cruder manifestations — take Britney and Madonna’s lip smacking, and the tonsil hockey of modern reality television — it’s a way to scandalize. But despite this breadth of meaning, we have very rigid ideas of what types of kissing are appropriate and acceptable — as Stephanie Seymour recently discovered after photos circulated of an ocean-side embrace with her son.