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来源：中国日报网 发布时间：2012-3-7
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5亿年前的一种形似蠕虫的生物被认为是所有脊椎动物的祖先。 
这种生物名叫皮卡虫，平均身长为5厘米，现已绝迹。 
该生物在被发现之初被归为无脊椎动物，但日前科学家们找到了它所具有的脊椎动物的特征。
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& }: [. Y& l" [据英国《镜报》3月5日报道，科学家们日前宣布，人类最古老的祖先是生活在5亿年前的一种形似蠕虫的生物。) F9 {- c6 s; N
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这种微小的生物名叫皮卡虫，有着最原始的脊骨和骨骼肌，是已知的、最古老的脊椎动物。因此，它可称得上是所有脊椎动物的祖先，包括鱼类、鸟类、两栖动物、爬行动物以及人类所属的哺乳动物。6 J( L- J; u) n+ l
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皮卡虫现已灭绝，平均体长为5厘米，仅见于伯吉斯页岩化石中。
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1909年，美国古生物学家查尔斯·D·沃尔克特在加拿大的落基山脉发现了伯吉斯页岩。页岩中有成千上万种化石，许多都保存得相当完好。有些化石清晰到肌肉、肠道和神经索都可以辨认的程度，成为研究寒武纪生物大爆炸的一个重要窗口。
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* l" ~; i' t: n" _. \% E% b皮卡虫首次发现于1911年，当时被归为水蛭或蚯蚓类的无脊椎生物。但科学家们日前发现，这种微小的生物拥有一条名叫脊索的柔韧棒，是一种原始脊椎。此外，他们还在皮卡虫体内找到了名叫肌节的骨骼肌和血管系统。6 k2 G3 r& R; J! y/ D5 _, u2 A4 I# G' U
7 A, z5 ?$ s% {0 x该研究负责人、剑桥大学的西蒙·康韦·莫里斯教授表示：“这次肌节的发现是我们长期希望找到的确凿证据……现在有了肌节、神经弧、脊索和血管系统，这些足以证明皮卡虫是地球上最为古老的脊椎动物。”- _ H( L5 p" {# t- R* {
2 f0 f% n7 `" Q: F0 ^3 \0 Z" z, I+ x8 a该研究的合作人、多伦多大学生态学和生物进化学的助理教授琼·伯纳德·卡伦认为：“我们的分析证明皮卡虫确有脊索，但真正让人兴奋的是发现了大量的骨骼肌，这些块状的肌肉组织是脊椎动物的重要特征。”
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6 d D! U6 m' O. K2 n9 i/ w皮卡虫的身体如鳗鱼般侧向扁平，由分布在脊索两侧的分段肌肉块组成，整体呈S型。该生物很可能是在海床附近通过身体的侧向来回扭动行进。
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3 ~, m# ]* c; v$ b这次成果得益于科学家们对114个皮卡虫化石的深入研究，为了能高精度还原这个“不及拇指长”的生物的细节，研究人员使用了包括电子显微镜在内的大量成像技术。2 L, F7 T" e& C7 c3 p m
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The worm that turned: 500million-year-old worm is our oldest ancestor The discovery is the smoking gun we have long been seeking, says scientist who led the study9 \" d/ K7 q( b2 E" h$ [( k, `. k/ p0 L
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 All right grandad? An artist's impression of the worm scientists believe is our oldest ancestor PA/University of Cambridge
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+ _* P& [' k, f2 P- fThe oldest human ancestor is a 500million-year-old worm-like creature no longer than a thumb.
' L5 c. B1 }1 V5 n: l- q7 _! APikaia gracilens is the most primitive known vertebrate and therefore the ancestor of all descendant vertebrates, including humans./ j( S7 K+ E2 r
The tiny creature had the beginnings of a backbone and skeletal muscle making it the oldest ancestor like us.7 w _5 T/ i( M2 ~' m2 r( g! }
It is a member of the chordate family, the group of animals that today includes fish, amphibians, birds, reptiles and mammals.
0 c1 g, W6 Q |. ] f+ XPikaia, now extinct, averaged about five centimetres in length and has been found only in the Burgess shale fossil beds of Canada’s Yoho National Park.
. M+ [' u) ]. ~7 V. QIt was first discovered in 1911 and thought to belong to the leech or earthworm family.
' ~5 `1 s/ K/ N$ ~1 c; fBut scientists now say it had a flexible rod called a notochord which was a primitive backbone. R3 y+ a8 s; F7 e3 I& G8 A
They have also found it had skeletal muscles called myomeres and a blood vessel system
! Z! B7 p- T# s) X( Q5 @5 R Fossils: Pikaia gracilens were about 5cm long PA/University of Cambridge
& H2 H& F0 C$ _# D Discovery: Pikaia gracilens are found in the Burgess shale fossil beds of Canada’s Yoho National Park PA/University of Cambridge 9 x# w$ }7 S# z5 d1 W. C0 j
“The discovery of myomeres is the smoking gun that we have long been seeking,” said the study’s lead author, Professor Simon Conway Morris of the University of Cambridge, lead author of a study published in the journal Biological Reviews.
2 @3 F1 M- M8 I" h% ~“Now with myomeres, a nerve chord, a notochord and a vascular system all identified, this study clearly places Pikaia as the planet’s most primitive chordate. So, next time we put the family photograph on the mantle-piece, there in the background will be Pikaia.”
+ l9 N& g% ?( E* l7 W4 w, K+ bPikaia was a sideways-flattened, eel-like animal. The flattened body is divided into a series of segmented muscle blocks seen as S-shaped lines that lie on either side of the notochord which runs along most, if not all, of the body length.
- R4 `- N! t4 g! s* W9 iIt probably swam above the sea floor by moving its body in a series of side-to-side curves.
# i; r1 u# I% j5 ]“Our analysis provides evidence that Pikaia indeed had a notochord,” said co-author, Jean-Bernard Caron, an assistant professor of ecology and evolutionary biology at the University of Toronto3 K. E# y8 b, Z/ N
“But the real excitement was finding extensive myomeres, the blocks of skeletal muscle tissue that are characteristic of chordates.
, I( D2 r9 j$ M“It’s very humbling to know that swans, snakes, bears, zebras and, incredibly, humans all share a deep history with this tiny creature no longer than my thumb.”
. W- a7 K# u+ i* z; vThe Burgess Shale is famous for its weird and wonderful fossils of marine organisms. The site provides vital information about evolution during the Cambrian explosion, a period over half a billion years ago that was characterized by the appearance of a vast diversity of animals over a short period of time., B9 n0 p- s" B# C: L& c
The study examined 114 Pikaia fossils using a range of imagery techniques, including scanning electron microscopy, to reveal fine details.
2 U2 g. d" }3 s: [- l“Pikaia would stand no chance of preservation in normal conditions outside exceptional sites like the Burgess Shale, professor Caron said.
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( w0 p7 ~7 n$ H" @& Bhttp://www.mirror.co.uk/news/technology-science/science/humans-are-descended-from-worm-which-lived-500-752043 n+ O. @7 j5 |. N
6 N( x( D4 o# `" R$ e【扩展阅读】6 V) R4 _) b5 C, s$ c% l
癌可能是我们最古老的进化祖先http://www.stemcell8.cn/thread-38172-1-1.html
: ^! ^0 O D% C/ c B奥地利科学家在祖先多细胞动物中发现“原始的”致癌基因 http://www.stemcell8.cn/thread-38310-1-1.html
; m9 o3 D1 x) C. P" w2 tMBE：所有遗传疾病的基因有同一“祖先”http://www.stemcell8.cn/thread-38156-1-1.html% T# ]( p. \0 d. q+ I6 K% G
标题理解错误：研究显示疟疾和海藻来自同一祖先 http://www.stemcell8.cn/thread-22348-1-1.html% ]# i6 N: O0 f/ `1 Y) w
癌——来自远古的鼓声 http://www.stemcell8.cn/thread-40136-1-1.html
6 R% }6 _/ j7 r) d. ~' K# L癌基因的远古起源：与后生动物的多细胞化相关 http://www.stemcell8.cn/thread-38158-1-1.html
0 u |4 `1 Y2 x, }) m$ b研究揭示远古病毒侵入影响人类基因组 http://www.stemcell8.cn/thread-27206-1-1.html
+ B+ \% y* e! B- Q永生的涡虫体细胞可对抗衰老 http://www.stemcell8.cn/thread-52972-1-1.html
; M1 M: m3 u+ s4 l& l涡虫可用于模拟胚胎干细胞实验 http://www.stemcell8.cn/thread-53031-1-1.html7 X1 i1 B% l( R; u9 @5 U' V* ?
永生的涡虫体细胞可对抗衰老 http://www.stemcell8.cn/thread-52972-1-1.html
( Q. s+ |. z! x% _1 U- X: `7 v涡虫扁虫：干细胞生物学和神经系统再生的体内模型 http://www.stemcell8.cn/thread-35700-1-1.html
: @' C8 [; N; g1 l9 t0 U英国科学家解开涡虫再生之谜 器官再生有望实现（附原文） http://www.stemcell8.cn/thread-20255-1-1.html8 _5 a! ~. o& L1 U* I9 \7 q
德国马克斯·普朗克分子生物研究所发现控制涡虫再生能力的“总开关”（附原文） http://www.stemcell8.cn/thread-19052-1-1.html
, j x: M1 j4 Q# P; t7 w古老基因助蠕虫具超强再生能力 http://www.stemcell8.cn/thread-40075-1-1.html3 o* ?' E: w8 c( P/ O
《cell》文献翻译:SKNy蠕虫和长寿 http://www.stemcell8.cn/thread-11780-1-1.html
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发表于 2012-3-8 10:04
发表于 2012-3-8 12:37
