Simplify the following text for a Grade 11 native English-speaking person.
Original: A study by Hey has indicated that of the people migrating across this land bridge during that time period , only 70 left their genetic print in modern descendants , a minute effective founder population -- easily misread as though implying that only 70 people crossed to North America .
Simplified: A study by Hey has indicated that of the people migrating across this land bridge during that time period , only 70 left their genetic print in modern descendants .