Hey WonChang!
That's a great question!
Although the word "that" is sometimes omitted, it MUST be included in some cases. The simple way to think about it is this:
When you have nonessential information ("including an increase in the number of genes related to sensory perception and heart function compared to other mammals") that appears after a place where the word "that" sometimes can be omitted, just know that you will always see the word "that" there. The reason is that the nonessential information breaks up the flow of the sentence, which naturally makes it harder for the reader to follow. As such, the word "that" must be included to make the information easier to read.
Hope this helps!
Keep Moving Forward! You Got This!
JinTeacher