生物谷報(bào)道：產(chǎn)卵哺乳動(dòng)物（又稱單孔類動(dòng)物）為澳洲所特有,，一般分為兩組，嘴巴像鴨的平足鴨嘴獸和矮胖的針鼴鼠,。由于缺乏化石證據(jù),，關(guān)于這兩組動(dòng)物的進(jìn)化分歧時(shí)間一直是個(gè)生物學(xué)難題。基因序列變異的檢測(cè)數(shù)據(jù)曾表明,，它們的分歧時(shí)間大約在1700萬(wàn)年前至8000萬(wàn)年前之間。
鴨嘴獸
（圖片來(lái)源：Punchstock）
美國(guó)科學(xué)家近日利用在澳洲發(fā)現(xiàn)的一種遠(yuǎn)古動(dòng)物下頜骨化石,，找到新的證據(jù)表明，這一分歧發(fā)生的時(shí)間可能要更早一些,，在一億多年之前。相關(guān)論文發(fā)表在美國(guó)《國(guó)家科學(xué)院院刊》（PNAS）上,。
這一化石屬于一種名為T(mén)einolophos的遠(yuǎn)古動(dòng)物，生活于一億多年前的澳洲,。美國(guó)德州大學(xué)奧斯汀分校的Timothy Rowe和同事，利用高分辨率的X射線斷層掃描技術(shù)對(duì)Teinolophos的頜骨進(jìn)行了分析,。
結(jié)果顯示，下頜骨中有一個(gè)較大的槽,，從形狀和尺寸來(lái)看,，都更像是鴨嘴獸而不是針鼴鼠,。在現(xiàn)代鴨嘴獸，這一槽為它們敏感的“鴨嘴”補(bǔ)給神經(jīng)和血液,。另外，對(duì)化石牙齒的檢查也揭示了更像鴨嘴獸的特征,。這意味著，鴨嘴獸和針鼴鼠的分歧時(shí)間應(yīng)該是在這種化石動(dòng)物出現(xiàn)之前,，也即一億多年之前。
這一結(jié)果與分子測(cè)序估測(cè)的結(jié)果存在差異,。Rowe說(shuō)：“看起來(lái)像是單孔類動(dòng)物具有一個(gè)緩慢的進(jìn)化史,。”如果確實(shí)如此，則意味著之前基因序列變異的檢測(cè)人員低估了變異積累所需的時(shí)間,。Rowe表示，單孔類動(dòng)物代謝速度慢,，代隔（generation times）時(shí)間長(zhǎng)，這二者從理論上來(lái)說(shuō)都可以降低進(jìn)化變異的速度,。他同時(shí)認(rèn)為，單孔類動(dòng)物也許根本不需要進(jìn)化,，因?yàn)樗鼈兊牟东C能力已經(jīng)調(diào)整得非常好，比如鴨嘴獸的“鴨嘴”具有高度敏感的神經(jīng)末梢,，用來(lái)偵測(cè)獵物釋放的電信號(hào)。
美國(guó)加州大學(xué)伯克利分校的進(jìn)化生物學(xué)家David Wake認(rèn)為,，這一發(fā)現(xiàn)確實(shí)很有意義。但他對(duì)結(jié)果抱有不同意見(jiàn),。他說(shuō)：“我不太相信它是鴨嘴獸,。如果一定要將它放在進(jìn)化鏈的某個(gè)地方,，那么目前只能認(rèn)為是鴨嘴獸，但是我們對(duì)其它存在過(guò)的遠(yuǎn)古單孔動(dòng)物一無(wú)所知,。”他提醒說(shuō)，將來(lái)的化石發(fā)現(xiàn)將會(huì)改變它的位置（科學(xué)網(wǎng) 梅進(jìn)/編譯
生物谷推薦原始出處：
Published online before print January 22, 2008
Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0705858105
NEUROSCIENCE
Action-blindsight in healthy subjects after transcranial magnetic stimulation
Mark Schram Christensen*,,, Lasse Kristiansen*, James B. Rowe,¶, and Jens Bo Nielsen*,||
*Department of Exercise and Sport Sciences, University of Copenhagen, Nørre Allé 51, DK-2200 København N, Denmark; Danish Research Centre for Magnetic Resonance, Copenhagen University Hospital, Hvidovre, Kettegård Allé 30, DK-2650 Hvidovre, Denmark; Department of Clinical Neurosciences, University of Cambridge, Addenbrooke's Hospital, Cambridge CB2 2QQ, United Kingdom; ¶Medical Research Council Cognition and Brain Science Unit, University of Cambridge, 15 Chaucer Road, Cambridge CB2 7EF, United Kingdom; and ||Department of Neuroscience and Pharmacology, University of Copenhagen, Faculty of Health Sciences, Panum Institute, Blegdamsvej 3, DK-2200 København N, Denmark
Edited by Lawrence Weiskrantz, University of Oxford, Oxford, United Kingdom, and approved November 27, 2007 2007 (received for review June 22, 2007)
Abstract
Clinical cases of blindsight have shown that visually guided movements can be accomplished without conscious visual perception. Here, we show that blindsight can be induced in healthy subjects by using transcranial magnetic stimulation over the visual cortex. Transcranial magnetic stimulation blocked the conscious perception of a visual stimulus, but subjects still corrected an ongoing reaching movement in response to the stimulus. The data show that correction of reaching movements does not require conscious perception of a visual target stimulus, even in healthy people. Our results support previous results suggesting that an efference copy is involved in movement correction, and this mechanism seems to be consistent even for movement correction without perception.
consciousness | perception | reaching | efference copy
