<p>Endosymbiosis plays a key role in shaping biodiversity, but its origin and maintenance remain difficult to explain. Vertical transmission of endosymbionts is an important mechanism that can stabilize endosymbiotic interactions, but it also restricts the rate at which new endosymbiotic units can be formed. To explain the prevalence of endosymbiotic interactions, additional mechanisms that could increase the rate at which novel endosymbiotic units are formed should be considered. Horizontal transfer of endosymbionts may increase the rate of endosymbiosis formation. The underlying assumption of this mechanism, that endosymbionts have evolved to escape their host actively, has not been tested experimentally. We tested this mechanism in ciliate and algal species that differ in their ability to form endosymbiosis. We found that algal cells escaped from the ciliate hosts, with endosymbiotic algal species escaping more frequently than those that are intracellular only as prey. We tested the advantage of this mechanism and found that the rate at which new endosymbiotic units are formed was three times higher when both vertical and horizontal transmission by escaped cells was possible, compared to vertical transmission alone. This, together with the finding that the escape of endosymbiotic algae was higher in endosymbiotic than in predatory ciliate species and higher in environments where endosymbiotic units grow slowly, suggests that the escape has evolved as a mechanism. This finding has important implications for the conditions favoring the evolution of endosymbiosis as this mechanism can increase the rate of novel endosymbiotic unit formation and promote more advantageous endosymbiotic partner pairings.</p>