Hi All

Metallicity might've peaked some billions of years ago, but more relevant might be that gamma-ray bursts get rarer after ~ 2-3 gigayears ago, and hence NOW and not THEN is a more biofriendly epoch. GRBs, whatever their cause, aren't friendly to any life that needs sunlight, directly or indirectly since they tend to destroy ozone-layers and pollute atmospheres with NOx compunds.

My suggestion: All the old bio-compatible planets have probably been singed as their stars run out of main sequence before GRBs decreased in frequency enough to let life get past being microbes. Even here on Earth photosynthetic organisms probably didn't get a leg up until ~ 2.5 Gya, hence the early Proterozoic oxidation event ~ 2.3 - 2.1 Gya. Even after oxygen began increasing it probably didn't climb to near present values until after the Iceball phase c. 750 Mya - 580 Mya.

For our own Sun the high-oxygen biosphere probably has ~ 0.5 - 1 Gyr left to it, and probably really began ~ 0.6 Gya. Hence the time for complex life is ~ 10 - 15% of the Main Sequence lifetime of 11 Gyr. Cooler stars have longer Main Seqeunce spans, but do they make enough high-frequency light for photosynthesisers to crack water in a couple of billion years like on Earth?

And if I'm correct all biocompatible planets entered a complex life friendly epoch at roughly the same time, hence ET is roughly at the same stage as us.

Adam