生物新科技让你的年龄无法隐瞒

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Radioactive eyes don't lie (about your age, anyway)

Danish scientists find that measuring the amount of radioactive carbon-14 in people's eye lenses can accurately tell when they were born.
By Thomas H. Maugh II, Los Angeles Times Staff Writer
February 23, 2008

Radioactive carbon-14 trapped in the lens of the eye permits researchers to accurately date the year of a person's birth, Danish scientists report.

The lens contains proteins, called lens crystallines, that are transparent, allowing light to pass through to the retina. These proteins are produced during the first year of life and are unchanged afterward, providing a unique record of the time of birth.

The only other bodily proteins that remain unchanged throughout life are those in the enamel of teeth, but they are formed over a five- to six-year period and are thus less useful in dating.

The key to the dating process is the incorporation into the crystallines of carbon-14, which is found in all the food a person consumes. The Earth's atmosphere normally has a fairly constant level of carbon-14: it decays into nitrogen-14 with a half-life of 5,730 years, but is constantly replenished by the interaction of cosmic rays with nitrogen-14.

Because it is taken up by plants, all living organisms have a concentration of carbon-14 equal to that in the atmosphere. When the organism dies, the isotope decays slowly until none of it is left -- the phenomenon that is used for radiocarbon dating of archaeological artifacts.

During the period of atmospheric testing of nuclear bombs between 1945 and 1960, the atmosphere received a sharp spike of carbon-14, commonly called the \"bomb pulse,\" that has gradually been declining to normal levels as excess carbon-14 has been absorbed into the ocean.

The yearly amount of carbon-14 has been carefully monitored and is well known.

Forensic medicine specialist Niels Lynnerup of the University of Copenhagen and physicist Henrik Kjaeldsen of Aarhus University reported Tuesday in the online journal PLoS One that measuring the amount of carbon-14 in the lenses of 13 corpses using a large nuclear particle accelerator provided the year of birth.

The primary restrictions on the technology are that the person has to have been born after 1950, the lens must be removed within three days after death before it decays too much, and the individual cannot have subsisted primarily on seafood, which raises the readings.

最近，丹麦科学家报道，测定人类眼球晶状体中放射性碳-14的含量可以帮助研究人员准确判断一个人的出生年份。

晶状体内含有一种叫晶状体晶体蛋白的透明蛋白质，允许外界光线从中穿过到达视网膜。晶状体晶体蛋白在生后第一年即产生，且以后不再发生改变，成为出生时间的一个特殊印迹。体内唯一另外一种在整个生命过程中不发生改变的蛋白是牙釉质蛋白质，但牙釉质蛋白的形成通常需要5年或者6年以上，因此对于确定出生时间来说，其意义相对较小。

人类所有食物中均可检测到碳-14，碳-14与晶状体结合是确定生物体年代的关键。通常地球大气层中碳-14水平保持相对恒定：大约经过5730年，才有一半碳-14衰变为氮-14，但宇宙射线与氮-14的相互作用可使碳-14含量不断得到补充。碳-14由植物吸收，因此所有活生物体内碳-14的含量都与大气相同。当生物体死亡，同位素逐渐衰变，直至消失。由于碳-14半衰期如此之长，使得碳-14成为考古学家眼中测定生物体年代的无价之宝。

1945年到1960年大气核弹试验期间，大气中碳-14含量迅速增加，通常被称为“脉冲炸弹”，随着过多的碳-14被海洋吸收，碳-14含量逐渐下降至正常水平。此后，碳-14含量每年进行严密监测，并已是人所共知。

哥本哈根大学法医学专家Niels lynnerup和奥胡斯大学物理学家Henrik Kjaeldsen在《PLoS One》网络版报道，利用大型核粒子加速器检测13具尸体晶状体中的碳-14含量，可以确定被检者的出生年份。

目前该技术的广泛应用还存在一些限制，例如，被检者必须是1950年以后出生，晶状体必须在死后3天内碳-14大量衰变之前取出，且死者生前不能以海产品为日常食物，否则会导致碳-14检测数值升高。