Would love for Sailor to weigh in
Be careful what you ask for; here goes
in chronological order, the various somewhat scientifically and objectively defensible possibilities for the beginning of human life:
1. Moment of conception - usually within 48 hours of ovulation
2. First cell division - within 48 hours of conception - the DNA of a unique human being has been formed
3. Attachment of zygote/embryo to the wall of the uterus - usually approximately 9 days after ovulation, but maybe anywhere from 6-12 days after ovulation; or, put another way, usually approximately 7 days after conception
4. First heartbeat - about 3-4 weeks after conception
5. Beginning of brain function - 40-43 days after conception
6. Viability - about 23 weeks after conception, iirc
Sorry 107, consciousness, being too slippery a concept, did not make the list
Interesting sidelight, St. Thomas Aquinas believed that human life began 40 days after conception because that was when the soul entered the body. The agreement with the beginning of brain function is probably just a coincidence because the number 40 was supposed during the Middle Ages to have great religious significance. But still pretty interesting, St. Thomas was very bright and intellectually innovative.
Obviously, all of these could have some objective criteria to support them as the beginning of human life. The beginning of brain function, at around 40 days, seems reasonable and interesting. In science we have brain death when brain functions cease, so why not have the beginning of human existence coincide with the beginning of brain function?
Open to other suggestions, backed up by reasonable supporting arguments of course. Or you can agree with one of these and explain why it is better than the others.