In the sentence provided, the pronoun that would serve as an antecedent for "Wangari Muta Maathai" is "she."
For example: "Wangari Muta Maathai became the first African woman to receive a Nobel Peace Prize because of her devotion to democracy, human rights, and conservation. She was a remarkable leader."
In this case, "she" refers back to "Wangari Muta Maathai."