We evolved to like sour foods, foods with complex flavors, and umami foods, I believe, so that we would eat more bacteria-laden food. Why do we need to eat such food? Perhaps to get enough Vitamin K2. Vitamin K1 and Vitamin K2 are quite different. A brief introduction:
The term vitamin K refers to a group of compounds that have a 2-methyl-1,4-naphtoquinone ring in common but differ in the length and structure of their isoprenoid side chain at the 3-position. The 2 forms of vitamin K that occur naturally in foods are phylloquinone (vitamin K1) and the group of menaquinones (vitamin K2, MK-n), which vary in the number of prenyl units. Whereas phylloquinone is abundant in green leafy vegetables and some vegetable oils, menaquinones are synthesized by bacteria; therefore, they mainly occur in fermented products such as cheese.
A 2004 study found a huge protective effect of K2:
The scientists at Osaka City University gave 21 women with viral liver cirrhosis [which greatly increases your chances of liver cancer] a daily supplement of 45mg vitamin K2 (menaquinone) for a period of two years. A group of 19 women with the disease received a placebo for the same time. Liver cancer was detected in only two of the 21 women given vitamin K2 but nine of the 19 women in the control group, reports the team in today’s issue of JAMA (292:358-361). After adjustment for age, severity of disease and treatment, the researchers found the women receiving vitamin K supplementation were nearly 90 per cent less likely to develop liver cancer.
A huge effect, suggesting that K2 is necessary for a repair system to work properly. This recent article is more support for the idea that K2 protects against cancer. The effect is weaker, perhaps because there was less damage needing repair.