贾第虫因具一系列 “简单“或”原始”的特征而被认为是最原始真核细胞。但也有认为这些特征应是其寄生生活退化所致。国际上争议颇盛，以致曾一度以该生物为模型的进化研究也陷入停滞。基于前期研究积累，我们认为：贾第虫很可能是分化出来极早的真核生物，后来走上了营寄生生活的演化之路而发生了一些次生的寄生适应性退化。因而现存的贾第虫是“极端原始性”与“次生寄生适应性”的“嵌合体”。本项目拟通过对贾第虫与其近缘的自由生活种类、以及与其寄生在相似环境的原虫之间进行系统的比较基因组学研究来验证该假说。主要检验：1）与寄生生活无直接关系、但为细胞的基本功能活动相关的基因和功能途径（如DNA的复制、转录和翻译系统等）是否仍具原始性；2）与适应寄生直接相关的方面（如组成吸盘的微管、能量代谢系统等）是否发生了寄生适应性“特化”或“退化”。从而甄别该生物在进化生物学研究中的真正价值之所在，推动相关研究领域的突破发展。

By studying primitive eukaryotic cells, the origin and evolution of the eukaryotic cell can be explored, and what is more important, it is helpful to understand why the present eukaryotic cells possess so ingenious and complex structures and functions, that is, it would improve our knowledge by from knowing how to knowing why. There are lines of evidence to prove that Giardia is such a kind of valuable primitive unicellular eukaryote. But some authors considered that the so-called primitiveness of Giardia is actually the simplification due to its parasitic degeneration. Based on the achievements of our long study, we propose a hypothesis which requires to be verified: Giardia is a combination of the extreme primitiveness and secondarily parasitic adaptation. Based on genome data, this project is going to compare Giardia with their free-living close relatives, and with other parasitic protozoans living in the similar environment to verify this hypothesis. Mainly checking whether the genes which are involved in the basic structures and functions of the cell but have no relationship with parasitism remain primitive, while those which have close relationship with parasitism of Giardia have undergone secondarily parasitic adaptation such as ‘specialization’ and ‘degeneration’. By carrying out this project, besides understanding the aspect of parasitic adaptation of Giardia, the key position of Giardia in the evolution of eukaryotic cells and its real value for the study in this field would be clarified, which could certainly promote the development of the two important fields.