如何计算一行中的数值

I have a data frame like this

我有这样的数据框

df <- as.data.frame(read.table(text =
                    "Human_Gene_Name    Human    Mouse    Chicken    Worm
                  Gene_1    8.5    7.0    NA    5.0
                  Gene_2    5.5    NA    NA    NA", header  = T))
df