Field Museum, Chicago
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。」
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。
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。
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。」
A Modern Stone Age Family? A Neanderthal’s Molar Suggests Early Weaning
May 27, 2013
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.
NatureTwo 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.