The article indicates the authors think this happened only about 100 million years ago, which is very recent compared to when mitochondria and chloroplasts became symbionts a billion+ years ago.
Another big difference here is that the chloroplast symbiosis occurred early in plant evolution and those symbionts became the common ancestors of all living plants. And mitochondria symbiosis was early enough to become the common ancestors of most living eukaryotes, such as plants, animals, and fungi.
These recent nitrogen fixing symbionts occurred too late to become established as a feature of a major branch of life, like plants or animals.
Considering that most species alive today have not yet been described and studied by scientists, there are sure to be many more interesting living biological organisms to be discovered.
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