Architecturally, the one square block of San Francisco bounded by Mission, Main, Howard and Spear streets isn’t much to look at. But for a crash course in how downtown’s fine-grain landscape evolves, it can’t be beat.

Colonnades are out, bike parking is in. Public spaces take all forms. Lobbies are in flux even as buildings stand still.

What you see, in essence, is the ongoing redefinition of urban life itself.

This might sound like a lot to ask of eight buildings, only one of which was built before 1973. The heights range from four to 29 stories. The biggest attraction for outsiders might be the Wells Fargo ATM at 100 Spear St.

The truly distinct element is tucked inside the block: a procession of four spaces that the public can visit during the workweek.

A pedestrian walks along a small urban trail nestled on a block bounded by Mission, Howard, Main and Spear streets in San Francisco. Photo: Jessica Christian / The Chronicle I’d call them corporate plazas, but they’re not. One is a courtyard alongside 123 Mission St., 29 stories of elongated travertine from 1986. The path widens to include concrete benches and small fountains behind the vaguely postmodern 135 Main St., which opened in 1990. Another few steps and a broad walkway shaded by redwood trees and lined with tables opens up on your left — the official entryway to 160 Spear St., even though that mundane 19-story chunk from 1984 actually is on Main Street.

If you keep going past the hint of forest you enter the final walkway, this one narrower, lined by boxed hedges and the brick backside of 188 Spear St. This passage — a “snippet” in planning language — runs alongside a warehouse-like office building that opened in 1973 but had four of its stocky 12 floors added in 2012.

Those extra floors are the only vertical change to the block in nearly 30 years. As for the mid-block pedestrian maze, it feels as spliced together as it sounds.

Back then, this was the Financial District’s new frontier — the far side of Market Street, attractive mainly to large firms that wanted space for employees who weren’t needed in the pricier headquarters tower.

San Francisco planners, meanwhile, were trying to humanize and revive a threadbare part of town left behind by economic changes. This impulse led to the 1985 Downtown Plan, which included a requirement that new commercial buildings had to provide publicly accessible private space. But the new guidelines weren’t yet in force.

“We knew what we viscerally thought would make a better city, but had no law to require it,” recalls Dean Macris, the city planning director in the 1980s. “So everything was subject to negotiation.”

High hopes aside, the mid-block realm these days tends to be used mostly for shortcuts or smoking breaks. The sign announcing “123 Mission Plaza Shops” leads to several empty storefronts.

Yet the block is anything but forlorn.

While the exteriors of the towers are stuck in a stodgy time warp, the lobbies have been updated with 21st century tenants in mind. Dark marble and clubby wood have given way to such airy lightness as the lobby at 135 Main St., with its waterwalls behind panels of translucent onyx. The public art inside the redone lobby at 188 Spear consists of patterned seating blocks that can be rearranged at will.

As for 160 Spear, the most drab of the lot, New York developer Tishman Speyer hired San Francisco’s IwamotoScott Architecture to send an abstracted ripple of triangular white ceiling panels through the lobby to the mid-block open space. The formulaic box also now sports a chic metal canopy that tilts out above the sidewalk.

A pedestrian walks through a small urban trail on a block in San Francisco’s Financial District. Photo: Jessica Christian / The Chronicle The canopy’s silvery sheen complements the ground-floor space along the lobby: the marketing center for Mira, the condominium tower that is being developed by Tishman Speyer now rising one block to the south.

The tower now rising is as flashy as 160 Spear and its neighbors are mundane, complete with a design by Chicago trendsetter Jeanne Gang that sends a tight corkscrew of angled bays 40 stories into the sky. It’s a contrast not just in styles, but in ideas about who might be attracted to this part of town.

“MIRA is supremely positioned in the heart of a bustling and vibrant neighborhood,” we’re told in the tower’s marketing brochure. Amenities will include a dog-washing station and refrigerated cubbies off the lobby for food deliveries. Prices for the 235 market-rate units start at $1.2 million.

The far side of Market is now a destination. And why not?

The Embarcadero’s viewtiful promenade is one block to the east, the double-decker freeway that scarred it long gone. A glass tower leased to Facebook is near completion to the west. The rooftop park above Transbay’s troubled transit center lies just beyond.

When I strolled toward Howard Street on the “snippet,” a door opened in 188 Spear and a young woman stepped out. Turns out a storage room inside has been converted for use as bicycle parking.

None of the changes on this block is dramatic. They don’t register on the skyline. Instead, they convey the subtle dynamics of a city like San Francisco — where prosperity is taken for granted and cultural trends are never static. And buildings, like people, adapt the best they can.