科学家说，C4草本植物很可能是通过从热带森林天棚之下扩散到更干燥的地区从而进化了出来。这项结果可能有助于解释C4光合作用的演化，一些植物进化出了这种更有效率的固定碳的途径，从而在高温、低二氧化碳的环境下把二氧化碳分解成氧气。C4植物覆盖了地球陆地面积的大约20%，包括重要的商业作物，如玉米、甘蔗和高粱，它的扩散被认为是地球历史上最重要的事件之一。
Erika Edwards和Stephen Smith把1230种草本植物的110万多植物标本的种系发生学、地理学、温度和降雨量数据结合起来，提出C4草本植物从密集的热带森林起源，并扩散到了零星的热带林地和稀树草原系统。这组科学家指出，C4植物过去已经存在于炎热的热带地区，并且在过去的3000万年中，随着植被从林荫扩散到更干燥、阳光更充足的环境中而进化出了C4光合作用途径。这组作者说，尚不清楚C4进化的发生是由于阳光直射产生了更高的叶片温度还是由于草本植物面临的节约水的压力增大。
推荐原始出处：
PNAS February 8, 2010, doi: 10.1073/pnas.0909672107
Phylogenetic analyses reveal the shady history of C4 grasses
Erika J. Edwardsa,1 and Stephen A. Smithb
aDepartment of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912; and
bNational Evolutionary Synthesis Center, Durham, NC 27705
Grasslands cover more than 20% of the Earth's terrestrial surface, and their rise to dominance is one of the most dramatic events of biome evolution in Earth history. Grasses possess two main photosynthetic pathways: the C3 pathway that is typical of most plants and a specialized C4 pathway that minimizes photorespiration and thus increases photosynthetic performance in high-temperature and/or low-CO2 environments. C4 grasses dominate tropical and subtropical grasslands and savannas, and C3 grasses dominate the world's cooler temperate grassland regions. This striking pattern has been attributed to C4 physiology, with the implication that the evolution of the pathway enabled C4 grasses to persist in warmer climates than their C3 relatives. We combined geospatial and molecular sequence data from two public archives to produce a 1,230-taxon phylogeny of the grasses with accompanying climate data for all species, extracted from more than 1.1 million herbarium specimens. Here we show that grasses are ancestrally a warm-adapted clade and that C4 evolution was not correlated with shifts between temperate and tropical biomes. Instead, 18 of 20 inferred C4 origins were correlated with marked reductions in mean annual precipitation. These changes are consistent with a shift out of tropical forest environments and into tropical woodland/savanna systems. We conclude that C4 evolution in grasses coincided largely with migration out of the understory and into open-canopy environments. Furthermore, we argue that the evolution of cold tolerance in certain C3 lineages is an overlooked innovation that has profoundly influenced the patterning of grassland communities across the globe.
