Complex sentence: Some proponents of this theory suggest they may be the descendants of Australian Aborigines who colonized the area prior to the arrival of mongoloid Amerindians., and this is lent credibility by research that suggests the existence of an ethnically-distinct population elsewhere in South America.
Split into simpler sentences: Some proponents of this theory suggest they may be the descendants of Australian Aborigines who colonized the area prior to the arrival of mongoloid Amerindians. Further credibility is lent to this idea by research suggesting the existence of an ethnically distinct population elsewhere in South America.