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When does a human life begin?

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Let me tell you. Is a complicated question but only slightly simpler than when does life ends. For most scenarios, the broad generalization is easily recognizable. Nonetheless, at the extremes of the spectrum, we are not quite sure. Everything that requires measurement leaves a small territory of penumbra known as uncertainty. Even after our best estimate posterior to multiple measurements.

TL;DR Don't leave without checking the sparks at the end, thank me later.

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Modified image Source 1 Source 2

In the case of the end of life, one of the points of controversy is the correct diagnosis of brain death. Most clinicians have to rely on the criteria gathered and compiled in evidence-based guidelines[1] Is considered an irreversible cessation of critical functions of a whole organism yet applying this to the criteria and properly recognizing it is more subjective than we would like to admit.

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Regardless of one's opinion about pregnancy and terminologies like "Baby" "Embryo" "Fetus", the whole being of human origin appears to be in the process of a new diploid life from the moment the zygote appears. The question is how precisely can we determine this moment.

At fertilization? seems too early. After the first mitosis? seems too late.

Isn't there an even more precise moment?

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Spermatozoids among their many particularities contain a strangely high concentration of zinc (tenfold when compared to any other cell in the body) and low zinc levels in the cell plasma have been correlated with infertility.[2]

Zinc is a necessary part of spermatogenesis but after that one can't help but wonder why the sperm cell needs so much zinc. The interesting part is how does this zinc behave after the egg and sperm cell unite.

What's even more interesting is that the levels of zinc in the human egg start increasing during maturation for ovulation, It had 40 billion atoms at arrest and gains, in close to 16 hours, 20 billion new atoms arriving at 60 billion (6 x 10^10) atoms of Zinc just at the moment is ready for contact with the cell sperm.[3]

Egg Loading Zinc, source

If you look closely at the image you will note that the granules of zinc are mostly at the periphery (cortical granules), sparing the zones where the chromosomes are.

When they finally meet (egg and sperm) something quite interesting starts happening. The levels of Calcium and Zinc start changing extremely fast. In cycles like pulses.[4]

Activated Human Eggs Spark, source

This is a single spark isolated in 3D, looks like a small world being bombarded by nukes. Sunspots if you prefer, that show the flux of metals that enable the egg to transition from egg to embryo.

Isolated Spark 3D

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[1] Wijdicks EF, Varelas PN, Gronseth GS, Greer DM, American Academy of Neurology; Evidence-based guideline update: determining brain death in adults: report of the Quality Standards Subcommittee of the American Academy of Neurology. Neurology. 2010;74(23):1911.

[2] Jiang Zhao, Xingyou Dong, Xiaoyan Hu, Zhou Long, Liang Wang, Qian Liu, Bishao Sun, Qingqing Wang, Qingjian Wu & Longkun Li; Zinc levels in seminal plasma and their correlation with male infertility: A systematic review and meta-analysis
Scientific Reports 6, Article number: 22386 (2016)

[3] Kim AM1, Bernhardt ML, Kong BY, Ahn RW, Vogt S, Woodruff TK, O'Halloran TV; Zinc sparks are triggered by fertilization and facilitate cell cycle resumption in mammalian eggs. ACS Chem Biol. 2011 Jul 15;6(7):716-23. doi: 10.1021/cb200084y. Epub 2011 Apr 28.

[4] Emily L. Que et al. Quantitative mapping of zinc fluxes in the mammalian egg reveals the origin of fertilization-induced zinc sparks. Nature Chemistry 7, 130–139 (2015)

Egg GIFs credit to Northwestern University by Dr. Emily Que