K–Pg boundary sections around the world can preserve an iridium anomaly and other impact indicators, including shocked minerals and spherules. The deposits vary by location; they are not one uniformly thin band with every marker present.

Together with the Chicxulub crater, the global evidence records a large asteroid impact about 66 million years ago. The boundary is not the whole catastrophe pressed into one line. It is a set of local sediments through which a global event can be correlated.

The layer is quiet rock, but impact models and deposits point to blast, seismic and ocean effects near the crater, ejecta travelling far beyond it, and atmospheric disruption that darkened and cooled the planet. Effects differed with distance and environment.

For living organisms, there was no single cinematic 'day after' shared everywhere. Food webs failed across months and years. Their world did not turn into ours; many lineages ended, others survived, and later evolution remained contingent.

Birds were already a living dinosaur lineage before the impact; some survived while every non-avian dinosaur lineage disappeared. Mammals also predated the boundary and diversified afterward alongside many other survivors.

The extinction changed ecological opportunity, but it did not happen in order to make humans possible. Our existence depends on countless prior contingencies; 'prerequisite' becomes misleading when it turns dependence into destiny.

At some outcrops, a person can span the K–Pg boundary with a fingertip. Yet 'below' and 'above' do not mean that dinosaurs, ammonites, mammals, birds, and flowering plants line up neatly against every hand specimen. Preservation has gaps, sedimentation rates differ, and several surviving groups crossed the line.

The thinness is still startling. A brief interval can separate fossil assemblages whose difference took generations of death and survival to unfold. The layer is not the largest possible violence made flat; it is one material trace through which that planetary change becomes measurable.