How would the body be affected if red blood cells had low levels of hemoglobin?

The body would not receive enough oxygen. Hemoglobin binds O2 and transports it throughout the body. At first, the body tissues would be hypoxic.

Don't know how much detail you need, but later, hormones would increase breathing rate and blood pressure to bring in more oxygen and pump blood harder. Also, different hormones would stimulate blood cell production.

then that particular organisms become extinct.

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