Even though there is no intelligent life, I believe that there is a presence of single-celled organisms.
Even though there is no intelligent life, there is evidence that supports that single-celled organisms are present.
一般的に、「～ではないにせよ」はEven though it isn'tやEven though there is noになります。
何かがそうじゃない場合、Even though it isn'tを使います。
例えば、
Even though it isn't a long trip, we still need to prepare.
長い旅行ではないにせよ、準備は必要だ。
何かがいない・存在しない場合、Even though there is no ○○と言います。
Even though there is no intelligent life, we believe that there is a presence of single-celled organisms.
知的生命体ではないにせよ、単純な構造の生き物なら存在すると思う
個人の意見だったら、I believe that there is a presence~と言います。
科学的な話で、「○○と思う」はwe thinkよりwe believe~やthere is evidence that supports that~の方が使われています。
そして、証拠があればeven though there is no intelligent life, there is evidence that supports that single-celled organisms are present. と言います。
ご参考になれば幸いです！
アメリカ合衆国 
