Not sure that my explanation makes sense

be 是係動詞， 表狀態（是或不是, 不存在‘正在是’）
Dog has been the most friendly animal to human since human started to domesticate wild animals.


walk 作為動詞， 可以講‘正在走’
Human have been walking using only two rear limbs since they were no longer apes.(一直用兩條腿走， 現在亦如此）