in classical 1（神话） , the 2（柏树） tree is associated with death, the underworld and 3. indeed, the family to which 4 belong, is an ancient lineage of conifers（松柏类） , and a new study of their evolution affords a unique insight into a turbulent era in earth's history. during the geological era known as the mesozoic, the 5 crust was concentrated in a single huge landmass, the supercontinent pangea. pangea began to break up about 150 million years ago, and the fragments drifted apart, eventually giving rise to the 6 of continents we know today. the progressive break-up of such a large landmass meant that existing groups of plant and animal species were split apart, and the descendant lineages then evolved in 7 from each other.
dating 9 with the 10 clock
"fossils show that the cypress family is a very ancient group of plants," says lmu biologist professor susanne renner, who is also director of the munich botanic garden. "we therefore suspected that it might be possible to follow their 11 history back to the period before the break-up of pangea, as long as the many episodes of climate change and associated extinctions had not obscured things too much." renner and her research group therefore set out to reconstruct the cypress family tree, based on the comparison of specific 12 sequences from 122 species belonging to 32 genera reflecting the family's worldwide distribution. in order to date divergence（分歧） events, they 13 the concept of the molecular clock.
the idea is based on a simple principle. when two lineages 8 from a common ancestor, each accumulates 14 substitutions independent from the other. to a first approximation, the number of unique substitutions（替换） provides a measure of the time that has elapsed since a species 15 from its sister species. by comparing the 16 of genetic changes found in different lineages and 17 the amount of change with fossils, one can therefore reconstruct a group's history.
evolutionary dead ends
"over the past 15 years, these molecular methods, in combination with new fossil finds, have revolutionized the study of biogeography, the branch of biology concerned with understanding the distribution patterns of animal and plant species," says renner. some groups have turned out to be surprisingly young in evolutionary terms, others much older than people had assumed.
the new study confirms that cypresses represent a very old plant family. their origins can be traced back to pangea, and the evolutionary divergence of the northern and southern subfamilies of cypresses actually reflects the break-up of pangea about 153 million years ago. as fragmentation progressed and ancestral lineages were separated from each other, new lineages were established and followed separate evolutionary 18. the cupressaceae is the first plant family whose evolutionary history gives us such a 19 picture of the break-up of a supercontinent（超大陆） .
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