2013年6月12日 星期三

A Revolution in the Making (WSJ)

A Revolution in the Making


Digital technology is transforming manufacturing, making it leaner and smarter—and raising the prospect of an American industrial revival
On a dark and stormy night two weeks ago in Schenectady, N.Y., Ken Hislop was relaxing at home when his cellphone suddenly began buzzing in his pocket. It was an urgent text message—from the General Electric Co. GE -0.84% factory where he works.

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Soon, a second message arrived. And then another, and another. The texts were being sent by tiny sensors embedded inside a series of machines, some of which look like enormous upside-down cement mixers. A violent thunderstorm passing through the area had caused something to go wrong.
"I knew right away we'd lost power at the plant," says Mr. Hislop, a manufacturing engineer. He quickly switched on his iPad and accessed animated schematic maps that signaled everything happening at the $170 million facility, which makes massive batteries for things like cellphone towers and power plants. Though the outage had been momentary, much of the equipment at the factory had to, in effect, reboot, and any blip could mean costly lost production time.
Ryan Etter
"I was getting a first-person, real-time account," says Mr. Hislop, who also could watch video of the storm from the plant's roof. The information allowed him to ensure that the machinery restarted in proper sequence and that the sensitive battery material hadn't been damaged.
Welcome to the New Industrial Revolution—a wave of technologies and ideas that are creating a computer-driven manufacturing environment that bears little resemblance to the gritty and grimy shop floors of the past. The revolution threatens to shatter long-standing business models, upend global trade patterns and revive American industry.
Impacts Big and Small
For big companies, it means a swath of new tools to build smarter, leaner factories and explore innovative new products, materials and techniques that weren't possible before. And thanks to plummeting prices, small companies have access to better, cheaper manufacturing equipment and design tools—giving even one-person startups the chance to create market-shaking innovations. Many people liken the era we're in to the early days of computing, where upstart hobbyists in their garages came up with huge advances that changed the industry. (See "Build a Better Mousetrap—Fast."
"Manufacturing is undergoing a change that is every bit as significant as the introduction of interchangeable parts or the production line, maybe even more so," says Michael Idelchik, who heads up advanced technologies at GE's global research lab, located about 15 minutes away from the battery plant. "The future is not going to be about stretched-out global supply chains connected to a web of distant giant factories. It's about small, nimble manufacturing operations using highly sophisticated new tools and new materials."
There's no question that a coinage like the New Industrial Revolution sounds magisterial, given the profound impact that the original Industrial Revolution had not just on business but on living standards around the world. And there's also no question that for all the big talk and big forecasts, many things will go on being produced using techniques that were all but perfected long ago.
But the big label is far from unwarranted. The upheaval, still in an early stage, is accelerating now thanks to the convergence of a number of trends: the low cost and accessibility of Big Data associated with cloud computing; the plummeting cost of electronic sensors, microprocessors and other components that can be used to make machines more adept; and advances in software and communications technology that make it possible to manage manufacturing with a whole new level of precision and enable new forms of collaboration.
A new wave of supercheap electronic sensors, microprocessors and other components means that facilities like Mr. Hislop's need almost no human help to do their jobs and can collect huge amounts of data along the way. Managers can get instant alerts about potential problems or study the numbers to find ways to boost efficiency and improve performance.

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The 3Doodler is a 3-D printer attached to a pen that quite literally lets you make three-dimensional doodles. Designed as a toy, it is being sought after by everyone from architects to the sight-impaired community.
Flexible Fabricating
At the same time, technological advances now allow manufacturers to invent new ways of fabricating things that represent an extreme departure from the classic production-line model. By far the most significant of these steps forward is additive manufacturing—a process of making a three-dimensional object of virtually any shape from a digital model.
These exotic machines can use a range of materials—everything from wood pulp to cobalt—and create things as varied as sneakers, fuel nozzles for airplanes and, ultimately, even human organs. And a single piece of manufacturing equipment, rather than being custom-designed to perform a single function, can be programed to fabricate a virtually limitless array of objects.
And, of course, that includes making more machines. On a tour of a laboratory of advanced manufacturing equipment that Autodesk Inc. ADSK -1.78% is building on a pier in downtown San Francisco, Chief Executive Carl Bass points to some masking tape on the ground that marks the spot where a sophisticated computer-controlled milling machine will be housed.
"The Japanese company Mori Seiki is making that in Sacramento in an automated factory," says Mr. Bass, whose company creates computer-aided-design software. "The factory is so advanced that you almost don't need to turn on the lights because the machines are doing everything, and what they are making is other machines." In fact, a 3-D printer has replicated itself at a university in England.
Still, manufacturers will have to navigate big new challenges in this era, too. For one thing, because additive manufacturing works from digital models of objects, companies are much more vulnerable to intellectual-property theft—the same way that easily copied music and movies have shaken the entertainment business.
The Sole of a New Machine
To get an up-close look at how the new technologies are already disrupting the old ways of doing things, consider Nike Inc.'s NKE -1.41% Flyknit shoe.
Nike
An upper for Nike's Flyknit shoe.
As high tech as some sneakers may be in materials and appearance, almost all of them are still made on assembly lines that put a shockingly heavy emphasis on human labor. Workers sit side by side in enormous facilities, cutting material and stitching and gluing shoe components together. But, starting last year, Nike began making the Flyknit a whole new way.
The company's engineers modified a machine used to make sweaters into a shoe-making contraption that knits the entire upper portion of the shoe in a single cocoon-like piece that is then attached to the tongue and to the sole. As the shoe is stitched, proprietary software instructs the machine to alter the materials being used—a bit more polyester thread here, a bit more there—to add strength or flexibility where needed.
Bespoke Products
A Bespoke Products leg.
Most important, it makes all these refinements at no added cost. The technology allowed Nike to make a shoe with just a few parts instead of dozens and with up to 80% less waste. "The Nike Flyknit is the world's first mass-produced consumer product made using additive manufacturing," says Maurice Conti, director of strategic innovation at Autodesk, which worked with Nike on the Flyknit project. "It's a hugely significant advance, not the least because, once you start doing things this way it obviously takes a lot of the labor cost out of the equation."
The implications for this are as obvious as they are profound: Almost seemingly out of the blue, the reason for making shoes in low-wage countries begins to evaporate and the advantages of locating the machine closer to the customer—in part for faster delivery—begin to loom much larger. Already, Adidas AG ADDYY -1.23% is knitting a shoe, the Primeknit, in its home country, Germany.
Last year, Boston Consulting Group published a report predicting that as much as 30% of America's exports from China could be domestically produced by 2020. President Obama gave a nod to this hope in his State of the Union address in February when he said that the popular additive-manufacturing technique called 3-D printing "has the potential to revolutionize the way we make just about everything."
Last year the president proposed a $1 billion addition to his fiscal 2013 budget to create a network of as many as 15 manufacturing-innovation institutes around the country. One is already up and running in Youngstown, Ohio, the setting of the Bruce Springsteen song about the rise and fall of the steel industry. Three more are in the works under the supervision of the Department of Energy and the Defense Department. Congress has yet to approve spending for the others.
Not So Fast
But the jury is out on whether a boost in manufacturing will create a resurgence in U.S. manufacturing employment, which peaked at around 19.5 million in 1979 and today totals around 12 million, according to the Bureau of Labor Statistics. (Economists attribute the recent modest increase in U.S. manufacturing employment to a rebound in the business cycle, and have found no evidence yet of an employment rebound connected to advanced manufacturing or the return of jobs from overseas.)
Almost certainly, it won't mean creating jobs the old way—building large factories that employ thousands of people. The real opportunity is in the growth of highly specialized, highly advanced microfactories and in legions of small entrepreneurial ventures making old things in new ways, as well as producing new products and custom-made items. An important sign of the times: the largest U.S. maker of 3-D printers, 3D Systems Corp., DDD -1.46% introduced a slick push-button model for $1,299 last year—putting it within range of the smallest businesses and home users. Kits to make a printer powered by software from the open-source RepRap project run as low as $400.
Experts envision bike shops that print custom frames and assemble bikes on demand; made-to-order shops or websites that offer one-off or personally designed jewelry; and more sophisticated production shops that crank out all manner of high-end products. Already, a company called Bespoke Products, a unit of 3D Systems, is making artificial limbs. Another, Organovo ONVO +1.22% Holdings Inc., is using 3-D printing to create human tissue for use in medical labs. At a recent conference, the company showed off a piece of raw meat it had made in a printer. Over time, this "democratization of manufacturing," as some refer to it, is expected to accelerate, and one day could mean that your local auto dealer or maybe even your neighbor (or you) will be able print out a replacement part for your car or make you a new cup holder sized perfectly for that enormous thermos you carry around.
New Ways of Making
Additive manufacturing may bring other changes that are just as dramatic as "factories" run out of somebody's garage. Additive manufacturing makes it possible to create designs or structures that weren't feasible using the two traditional ways of making things: milling (sculpting material out of a solid block) and casting (pouring liquid material that hardens into a mold). Both of these techniques are greatly enhanced by mass production because quality typically rises and costs fall as volume increases. Making a lot of something also means it's not so painful to discard defective units.
Advanced Architecture with Joris Laarman Lab
Mataerial, a prototype 3-D printer that can make curved objects.
But additive manufacturing enables the creation of materials with multiple parts and moving components without assembly. And because the process is entirely controlled by computers, following precise digital instructions, the very first piece that's manufactured is just as good as the last one. The incremental cost of producing a part becomes strictly a function of time and materials.
All of which means manufacturers can scan further afield for inspiration. Designers and engineers at General Electric have begun looking at ancient objects and prehistoric bird skeletons, and delving anew into topology, for inspiration on new forms of design. Their thinking: Centuries of making things under the constraints of old methods may have caused their predecessors to discard innovative structures simply because there was no practical way to produce them through milling or casting. But what was impractical in the past may be quite feasible today.
There's another big change playing out that isn't so obvious but could have a huge impact on the world of manufacturing. The rise of the 3-D printer has coincided with the digitization of the physical world through the use of 3-D scanners and, increasingly, two-dimensional photos that can be stitched together digitally using software to create precise 3-D renditions of anything made of atoms.
That affects everyone who works with manufacturers and who participates in the creative process: designers, engineers, materials specialists, machine makers and supply managers, among others. It's much easier to collaborate on a model if it is stored on a computer, because lots of digital hands can be working on it at the same time.
"The big untold story in all of this is the way the digitization of manufacturing compresses everything—from the early design of a product to its final assembly," says Ping Fu, who founded a company called Geomagic that makes 3-D modeling software and is now in charge of strategy at 3D Systems. "Everyone can now work together simultaneously. The software makes it possible, and you get much better results than when all of these activities were being done in different silos."
Still, this new environment leaves manufacturers facing big new challenges, as digital files of physical objects show up in huge numbers on websites like Thingiverse and Physibles, and manufacturing instructions appear online, too.
"I give a lot of speeches about this topic to manufacturing groups, and people are usually quiet during the Q&A," says Christine Furstoss, who oversees a staff of 450 engineers and scientists working on materials, energy strategy and processing technology at GE's research center. "But afterward, they come up to me in private and want to talk about how frightened they are. People get a glimpse of how this could change the game in their business, and they are just not sure what to do about it."
The Road Forward
For an idea of how the New Industrial Revolution might play out on a large scale, look at GE. Its footprints are everywhere in the advanced-manufacturing community. It is a highly visible participant in the federal government's efforts to boost additive manufacturing, as well as university programs focusing on the topic. Partners include the Massachusetts Institute of Technology, Amazon.com's AMZN -2.24% Web-services department and the Defense Advanced Research Projects Agency, which are collaborating with GE on a new crowdsourcing platform for product design and development.
General Electric
High-tech batteries from GE.
The company also is latching onto the technique in-house. For instance, it is making a big bet on additive manufacturing as a way to create engine parts that weigh less, cost less and employ more intricate designs. Last year, it bought one of the largest additive manufacturers in the U.S., Morris Technologies, and plans to use the company to make the sophisticated fuel nozzle for its next-generation jet engine, the LEAP. (The Morris family has a long industrial pedigree: It once supplied steel tubing to the Wright Brothers' bicycle shop.)
The new nozzle will be 3-D printed as a single part rather than assembled from 18 pieces, and it will be up to five times more durable. GE is also running its own 3-D metal printers, testing the procedure out on as many parts as possible for both the LEAP and the GE 9x, its next-generation 777 engine. This week, GE plans to announce a major investment in an another new additive-manufacturing factory that will mass-produce ceramic engine shrouds.
All told, the company projects it will spend $3.5 billion on aviation-related advanced manufacturing in the next five years and will produce 100,000 end-use parts for its engines annually by 2020 using additive techniques.
One of GE's most creative initiatives is an arrangement that will begin to make its more than 30,000 patents available to inventors and entrepreneurs who use the website Quirky—which employs crowdsourcing to evaluate ideas for products. "It's a whole new paradigm for innovation," says Ben Kaufman, the founder of Quirky, an industrial-design company in New York.
Starting this month, inventors and their ilk will be able to sift through the first 200 of GE's patents posted on Quirky, with more than 1,000 expected to be available by the end of the year. People who think they can use the technology without infringing on GE's own use will be able to click a button and begin a process enabling them to license use of the patent for whatever application they've dreamed up.
GE's efforts also offer a look at how data can be leveraged in this new era. One of the take-aways from a visit to GE's battery plant back in Schenectady, located adjacent to a parcel that housed Thomas Edison's machine works, is the sheer volume of data it generates—information that allows plant engineers to continually improve the production process and head off problems before they become serious.
The company can trace a product's entire genealogy, from containers of dirt, sand and salt to a bank of high-tech batteries supporting a nation's electric grid. The data not only improve quality control—if a defect shows up at any point, GE can trace it back to its original source—but in the end give GE a powerful competitive weapon that's virtually impossible to duplicate.
The Schenectady plant, nestled in a valley alongside the Mohawk River, is so extensively networked and connected, in fact, that it might just as easily be thought of as a single machine rather than a collection of them. And, of course, because it is so automated, it doesn't require a whole lot of human assistance. GE's Schenectady campus once had so many employees it was given its own ZIP Code, 12345. Yet even when it reaches full production—GE expects its output to exceed $1 billion in annual sales by 2020—the showcase battery plant won't employ more than 450 people.
Mr. Hislop, who confesses to using his iPad to check in on the factory during a recent camping trip, describes his experience on the night of the storm in the tones of an anxious parent. Yet in the midst of the howling winds and thunderclaps, the technology meant he could remain intimately in touch with everything that was happening across town. Despite the beating the plant was taking, he says he felt "reassured."
Mr. Koten is a columnist for WSJ.Money magazine in New York. He can be reached at reports@wsj.com.

2013年6月3日 星期一

一顆尼安德特人的牙齒窺探母乳餵養奧秘

一顆古人類牙齒窺探母乳餵養奧秘

研究者們從一名尼安德特人的牙齒得出結論,這名嬰兒的母乳餵養斷奶時間比靈長類動物和絕大多數現代人類都要早。
Field Museum, Chicago
研究者們從一名尼安德特人的牙齒得出結論,這名嬰兒的母乳餵養斷奶時間比靈長類動物和絕大多數現代人類都要早。
現代母親喜愛探討母乳餵養該持續多久這個能同時激起內疚和驕傲的話題。目前在一項非常初步的研究結果中,尼安德特人(Neanderthal)正被卷進這一話題。
通過分析一名尼安德特兒童臼齒化石中的鋇含量,研究者們得 出一個結論,這名兒童在最初7個月里是純母乳餵養的,接下來的7個月是在母乳之外輔以其他食物。科學家們周三在《自然》期刊(Nature)的網站上報告 說,隨後牙釉質中鋇的含量分佈「回到了產前的基礎水平,顯示出母乳餵養在1.2歲時突然停止。」
  • 檢視大圖 這名尼安德特兒童臼齒化石的兩幅切面圖。在上方的圖中,牙釉質的壓力線以藍線和數字標出,顯示齣兒童在有生之年的生長的顯著邊界。在下方圖中,四個區域顯示出了鋇濃度的不同,這種方法被用來檢測兒童飲食的改變。
    Nature
    這名尼安德特兒童臼齒化石的兩幅切面圖。在上方的圖中,牙釉質的壓力線以藍線和數字標出,顯示齣兒童在有生之年的生長的顯著邊界。在下方圖中,四個區域顯示出了鋇濃度的不同,這種方法被用來檢測兒童飲食的改變。

這個時間表符合美國兒科醫學會(American Academy of Pediatrics)的建議,即母親應對兒童進行6個月的母乳餵養,如果可能的話應持續到12個月,這比猿類和大多數現代人類的母乳餵養時間跨度都短。 工業革命前人口斷奶的平均年齡約2.5歲;而野生黑猩猩是5.3歲。當然,我們在演化上的表親尼安德特人所處的生活環境截然不同,他們已經滅絕了3萬年。
這些研究結果是要強調牙齒中的鋇含量與飲食變化之間的關 聯,這引起了一些科學家的強烈質疑。在《自然》的報告中,來自美國和澳大利亞的研究者描述了對人類嬰兒和圈養獼猴進行的測試,這些測試顯示出牙釉質中的鋇 元素似乎能夠準確地反映出母乳餵養中斷的飲食轉變。鋇元素的水平會在母乳餵養期間上升,並在斷奶後急劇下滑。
研究者們隨後決定把鋇測試應用到在比利時收集到的尼安德特兒童的臼齒化石上。研究者們發現,與他們擔心的不同,這枚牙齒的化石化並未破壞鋇元素生物標記。
研究者在接受採訪時說,這是第一例關於幼年尼安德特人飲食變化的資料,對於古人類化石的幼年飲食歷史來說,這預示着鋇測試技術或許能為進行更加嚴格的探索打開一扇門。
來自悉尼大學(University of Sydney)的研究團隊成員馬尼什·阿羅拉(Manish Arora)說,「我們對獼猴和現代人類兒童的研究提供了強有力的證據,表明牙齒中的鋇含量分佈準確地反映出了母乳斷奶的飲食轉變。」他還隸屬於紐約市西 奈山醫院(Mount Sinai Hospital)的伊坎醫學院(Icahn School of Medicine),並且是這些研究者的主要發言人。
但是德國萊比錫馬克斯·普朗克演化人類學研究所(Max Planck Institute for Evolutionary Anthropology)的古代牙齒和骨骼專家邁克爾·理查茲(Michael Richards)指出,對於考古樣本中鋇之類微量元素的檢測在上世紀七、八十年代就已經被廢棄不用了,因為科學家證明骨頭和牙齒會從埋葬它們的土壤中吸 收元素,並不一定來自活着的時候的飲食。
「最近,或許是得出這一成果的一代人退休了,」理查茲接著說,新一代人「又開始使用這些方法了。」他說他為《自然》期刊發表這篇報道感到吃驚。
其他研究尼安德特人和其他已滅絕人類遠祖的科學家對於他們研究結果的分析持謹慎態度。他們表示了擔憂,原因是研究中的關鍵元素取自僅僅一個化石標本。
阿羅拉承認,「當然,不可能把從一個樣本上得出的結論推廣到全部尼安德特人身上,但我們(對於一名年幼尼安德特人)的純母乳餵養期觀察確實拓展了現存的對於尼安德特人行為的認知。」
聖路易斯華盛頓大學(Washington University in St. Louis)的古人類學家埃里克·特林考斯(Erik Trinkaus)是一名尼安德特人研究領域的權威人士,他說,測試中斷奶的開始似乎太早。他還提醒道,「在我的印象中,哺乳的生理學和化學要複雜得多, 而且鋇的濃度太低,很難得到可靠的數據。」
哈佛大學演化生物學家塔尼婭·史密斯(Tanya Smith)是報告的合著者之一,她在一封郵件中說,該研究小組希望「分析更多的化石來確定嬰兒期的尼安德特人會在什麼年齡自然斷奶。」在報告中,研究者們承認,突然的、可能過早的斷奶要解釋起來並不容易。「
「我們對這項技術感到很興奮,因為我們感到,它讓我們可以通過更多的尼安德特人和智人的化石樣本直接觀察到斷奶這個生命歷史中的重要方面,」史密斯說。
斷奶時間對於現代人類社會有着重要的意義。斷奶時間過早會讓孩子面臨眾多健康問題的威脅;但較短的母乳餵養時間也意味着生育間隔時間的縮短,這會對人口增長速度產生影響。
翻譯:林蒙克

A Modern Stone Age Family? A Neanderthal’s Molar Suggests Early Weaning

Researchers have concluded, from the tooth of one Neanderthal child, that the infant was weaned off of its mother’s milk earlier than primates and a vast majority of modern humans.
Field Museum, Chicago
Researchers have concluded, from the tooth of one Neanderthal child, that the infant was weaned off of its mother’s milk earlier than primates and a vast majority of modern humans.

Modern mothers love to debate how long to breast-feed, a topic that stirs both guilt and pride. Now — in a very preliminary finding — the Neanderthals are weighing in.
By looking at barium levels in the fossilized molar of a Neanderthal child, researchers concluded that the child had been breast-fed exclusively for the first seven months, followed by seven months of mother’s milk supplemented by other food. Then the barium pattern in the tooth enamel “returned to baseline prenatal levels, indicating an abrupt cessation of breast-feeding at 1.2 years of age,” the scientists reported on Wednesday in the journal Nature.
  • 檢視大圖 Two views in cross-section of the Neanderthal child's fossilized molar. At top, stress lines in the tooth's enamel, denoted by the blue lines and numbers, show significant boundaries in the child's development in days lived. Below it, the four regions show barium concentration, a means to measure changes in the child's diet.
    Nature
    Two views in cross-section of the Neanderthal child's fossilized molar. At top, stress lines in the tooth's enamel, denoted by the blue lines and numbers, show significant boundaries in the child's development in days lived. Below it, the four regions show barium concentration, a means to measure changes in the child's diet.

While that timetable conforms with the current recommendations of the American Academy of Pediatrics — which suggests that mothers exclusively breast-feed babies for six months and continue for 12 months if possible — it represents a much shorter span of breast-feeding than practiced by apes or a vast majority of modern humans. The average age of weaning in nonindustrial populations is about 2.5 years; in chimpanzees in the wild, it is about 5.3 years. Of course, living conditions were much different for our evolutionary cousins, the Neanderthals, extinct for the last 30,000 years.
The findings, which drew strong skepticism from some scientists, were meant to highlight a method of linking barium levels in teeth to dietary changes. In the Nature report, researchers from the United States and Australia described tests among human infants and captive macaques showing that traces of the element barium in tooth enamel appeared to accurately reflect transitions from mother’s milk through weaning. The barium levels rose during breast-feeding and fell off sharply on weaning.
The researchers then decided to apply the barium test to the fossilized molar of a Neanderthal child, collected in Belgium. The tooth’s fossilization, the researchers discovered, had not destroyed the barium biomarker, as had been feared.
This is the first documentation of diet transitions in a juvenile Neanderthal, the researchers said in interviews, suggesting that the barium technique may open the way to a more rigorous exploration of early-life dietary history of fossil hominins.
“Our studies on macaques and modern human children provide strong evidence that barium patterns in teeth do accurately reflect transitions from maternal milk to weaning,” said Manish Arora, a team member from the University of Sydney. He is also affiliated with the Icahn School of Medicine at Mount Sinai Hospital in New York City, and acted as the principal spokesman for the researchers.
But Michael Richards, a specialist in ancient teeth and bones at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, noted that the examination of trace elements, like barium, in archaeological samples went out of use in the 1970s and ’80s, as scientists showed that bone and teeth incorporated elements from the soil they were buried in, not necessarily from a lifetime diet.
“Recently, perhaps as the generation that did this work retires,” Dr. Richards continued, a new generation has been “returning to these methods.” He said he was surprised that Nature published the report.
Other scientists who investigate Neanderthals and other extinct hominins were guarded in their assessment of the findings. They worried that the key element in the study was confined to only one fossil specimen.
Dr. Arora acknowledged that “it is, of course, not possible to generalize to all Neanderthals from a single sample, but our observation of the exclusive breast-feeding period” in one young Neanderthal “does extend existing concepts of Neanderthal behavior.”
Erik Trinkaus, a paleoanthropologist at Washington University in St. Louis, who is an authority on Neanderthals, said the onset of weaning in the test appeared to be too early. He also cautioned, “My impression is the physiology and chemistry of nursing is vastly more complicated, and the concentrations of barium are too low that it’s hard to get reliable data.”
Tanya Smith, an evolutionary biologist at Harvard and an author of the report, said in an e-mail that the team hoped “to examine additional fossils to determine at what age Neanderthals naturally weaned their infants.” In the report, the researchers conceded that the abrupt, possibly early weaning could not be readily explained.
“We are excited about this technique as we feel that it will allow us to look directly at weaning, an important aspect of life history, in expanded samples of Neanderthals and fossil Homo sapiens,” Dr. Smith said.
The timing of weaning can be critical in contemporary human societies. Completed too early, it can expose a child to more health problems; but shorter periods of breast-feeding lead to shorter intervals between births, which influences population growth.