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embryos of medaka fish. We found that most of <strong>the</strong>
genomic DNA is highly methylated but loci where H3K4
<strong>and</strong>/or H3K27 methylation occur are exceptionally
hypomethylated. Among <strong>the</strong>m, ~200 loci were found <strong>to</strong>
possess long DNA hypomethylated domain (>5kb) with
bivalent his<strong>to</strong>ne modifications. Interestingly, ~75 % of
genes of such loci are classified as key developmental
genes including transcriptional regula<strong>to</strong>rs such as
homeobox <strong>and</strong> T‐box genes. The subsequent analysis
using adult liver <strong>and</strong> muscle revealed that those key genes
are subject <strong>to</strong> DNA methylation, except for H3K4me‐rich
promoter regions, when actively transcribed. Our
time‐course data suggest that decrease in H3K27me
occurs first, prefiguring DNA methylation at later stages.
Toge<strong>the</strong>r with detailed examination on some specific
genes, we propose that key developmental genes are
regulated in a step‐wise manner throughout life, bivalent
<strong>and</strong> DNA hypomethylated state in undifferentiated early
embryos, his<strong>to</strong>ne‐modification‐dependent induction
during development <strong>and</strong> finally <strong>the</strong> maintenance of
transcription by DNA‐methylation after birth.
Evolution as a scientific <strong>the</strong>ory: evidence
from remarkable Chinese fossils
Zhonghe ZHOU
Institute of Vertebrate Paleon<strong>to</strong>logy <strong>and</strong> Paleoanthropology, CAS,
Beijing, China. Email: Zhonghe@yeah.net;
zhouzhonghe@ivpp.ac.cn
When Darwin wrote On <strong>the</strong> Origin of Species(1859), <strong>the</strong>
fossil record was poorly sampled <strong>and</strong> largely incomplete,
yet <strong>the</strong> knowledge of paleon<strong>to</strong>logy <strong>and</strong> geology cultivated
in <strong>the</strong> nineteen century played a fundamental role in
affirming Darwin’s idea of evolution. In <strong>the</strong> 1930s–1940s,
Simpson brought paleon<strong>to</strong>logy in<strong>to</strong> <strong>the</strong> evolutionary
syn<strong>the</strong>sis that constitutes modern evolutionary <strong>the</strong>ory.
With strong scientific backing, he established <strong>the</strong> study of
large‐scale evolution, or macroevolution, distinct from
evolution in a general sense (or microevolution). Ano<strong>the</strong>r
major <strong>the</strong>oretical contribution by paleon<strong>to</strong>logists
occurred in <strong>the</strong> 1970s, when Aldridge <strong>and</strong> Gould
proposed <strong>the</strong> <strong>the</strong>ory of punctuated equilibrium <strong>to</strong>
challenge <strong>the</strong> gradualism <strong>the</strong>ory of evolution.
Fossils provide <strong>the</strong> only direct evidence of <strong>the</strong> his<strong>to</strong>ry of
evolution. Fossils that fill <strong>the</strong>oretical evolutionary
intervals or gaps between existing taxa are often called
‘missing links’. In <strong>the</strong> 1850s, missing links were rare
<strong>and</strong> major gaps existed in <strong>the</strong> fossil record; however
<strong>to</strong>day, more than a century later, this is no longer <strong>the</strong>
case.
Fossil discoveries in <strong>the</strong> past 2 decades, particularly
from China, have greatly enriched our underst<strong>and</strong>ing of
<strong>the</strong> evolutionary tree of life <strong>and</strong> enhanced <strong>the</strong> evidence
in support of Darwin’s evolutionary <strong>the</strong>ory. Probably
most remarkable among <strong>the</strong>se discoveries are <strong>the</strong>
earliest vertebrates uncovered from <strong>the</strong> Early Cambrian,
<strong>the</strong> oldest <strong>and</strong> most primitive turtle from <strong>the</strong> Middle
Triassic, <strong>and</strong> four‐winged dinosaurs that are among <strong>the</strong>
closest relatives <strong>to</strong> birds. Fur<strong>the</strong>rmore, studies on
Chinese fossils have shed new light on major patterns of
biological radiation <strong>and</strong> extinction events during <strong>the</strong> last
500 million years.
The sudden appearance of advanced animals in <strong>the</strong>
Cambrian fossil record, known as ‘Darwin’s headache’,
can now be addressed through discoveries from various
early Cambrian <strong>and</strong> Precambrian lagerstätte; <strong>the</strong>
discovery of numerous flowered plants from <strong>the</strong> Early
Cretaceous also helps <strong>to</strong> explain Darwin’s ‘abominable
mystery’. Yet probably <strong>the</strong> most significant of <strong>the</strong>se
Chinese discoveries is <strong>the</strong> discovery of 160 million year
old fea<strong>the</strong>red dinosaurs that constitute perfect missing
links between birds <strong>and</strong> <strong>the</strong>ir dinosaurian ances<strong>to</strong>rs.
One of <strong>the</strong>se important taxa is named Anchiornis huxleyi,
with its species name in honor of Huxley, Darwin’
bulldog, who was <strong>the</strong> first <strong>to</strong> propose <strong>the</strong> hypo<strong>the</strong>sis of
a dinosaurian origin of birds.
Evolution as a scientific <strong>the</strong>ory or fact, has withs<strong>to</strong>od
various challenges over <strong>the</strong> past 150 years; however,
<strong>the</strong> <strong>the</strong>ory itself is still evolving <strong>and</strong> probably can never
be called complete. While paleon<strong>to</strong>logy continues <strong>to</strong>
contribute evidence of <strong>the</strong> fact of evolution <strong>and</strong> elucidate
details of evolutionary his<strong>to</strong>ry, we remain a long way
from filling <strong>the</strong> gap between our underst<strong>and</strong>ing of
evolution on <strong>the</strong> micro‐ <strong>and</strong> macroevolutionary scale.
Evolvability in sequence space
Daniel L HARTL
Department of Organismic <strong>and</strong> Evolutionary Biology, Harvard
University, Cambridge, MA 02138, USA. Email:
dhartl@oeb.harvard.edu
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